diff --git a/bincio/extract/simplify.py b/bincio/extract/simplify.py
index de9488d..41b184d 100644
--- a/bincio/extract/simplify.py
+++ b/bincio/extract/simplify.py
@@ -2,11 +2,51 @@
 
 from typing import Optional
 
-from rdp import rdp
-
 from bincio.extract.models import DataPoint
 
 
+def _rdp_mask(coords: list[list[float]], epsilon: float) -> list[bool]:
+    """Pure-Python RDP — returns a boolean keep-mask of the same length as coords."""
+    n = len(coords)
+    if n < 2:
+        return [True] * n
+
+    mask = [False] * n
+    mask[0] = mask[-1] = True
+
+    stack = [(0, n - 1)]
+    while stack:
+        start, end = stack.pop()
+        if end - start < 2:
+            continue
+        x1, y1 = coords[start]
+        x2, y2 = coords[end]
+        dx, dy = x2 - x1, y2 - y1
+        seg_len_sq = dx * dx + dy * dy
+
+        max_dist = -1.0
+        max_idx = start + 1
+        for i in range(start + 1, end):
+            x0, y0 = coords[i]
+            if seg_len_sq == 0:
+                d = ((x0 - x1) ** 2 + (y0 - y1) ** 2) ** 0.5
+            else:
+                t = ((x0 - x1) * dx + (y0 - y1) * dy) / seg_len_sq
+                t = max(0.0, min(1.0, t))
+                px, py = x1 + t * dx, y1 + t * dy
+                d = ((x0 - px) ** 2 + (y0 - py) ** 2) ** 0.5
+            if d > max_dist:
+                max_dist = d
+                max_idx = i
+
+        if max_dist >= epsilon:
+            mask[max_idx] = True
+            stack.append((start, max_idx))
+            stack.append((max_idx, end))
+
+    return mask
+
+
 def simplify_track(
     points: list[DataPoint],
     epsilon: float = 0.0001,
@@ -21,7 +61,7 @@ def simplify_track(
         return [p for p, _, _ in gps_pts]
 
     coords = [[lon, lat] for _, lat, lon in gps_pts]
-    mask = rdp(coords, epsilon=epsilon, return_mask=True)
+    mask = _rdp_mask(coords, epsilon=epsilon)
     return [p for (p, _, _), keep in zip(gps_pts, mask) if keep]
 
 
@@ -40,7 +80,7 @@ def preview_coords(
 
     # Coarse RDP (larger epsilon = fewer points)
     coords = [[lon, lat] for lat, lon in gps]
-    mask = rdp(coords, epsilon=0.001, return_mask=True)
+    mask = _rdp_mask(coords, epsilon=0.001)
     reduced = [gps[i] for i, keep in enumerate(mask) if keep]
 
     # Subsample if still too many — always include last point without exceeding max_points
diff --git a/pyproject.toml b/pyproject.toml
index 6099e03..69dabb5 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -18,8 +18,6 @@ dependencies = [
     "lxml>=5.0",          # TCX (XML)
     # Data
     "pandas>=2.2",
-    # Geo
-    "rdp>=0.8",
     # Config & CLI
     "pyyaml>=6.0",
     "click>=8.1",
diff --git a/site/public/sw.js b/site/public/sw.js
new file mode 100644
index 0000000..df4cf7c
--- /dev/null
+++ b/site/public/sw.js
@@ -0,0 +1,110 @@
+/**
+ * BincioActivity Service Worker
+ *
+ * Intercepts requests for /data/* and serves from IndexedDB when local
+ * activities are present, merging them with the bundled static index.
+ *
+ * IndexedDB schema:
+ *   db: 'bincio', store: 'files'
+ *   key: file path (e.g. '/data/activities/2024-01-01T120000Z-ride.json')
+ *   value: { path, data }  — data is the parsed JSON object
+ *
+ * Local activity summaries are kept under the special key '/data/local-index'.
+ * The SW merges these with the static index.json at request time so that
+ * activities from the server and from the device appear together in the feed.
+ */
+
+const DB_NAME = 'bincio';
+const DB_VERSION = 1;
+const STORE = 'files';
+
+// ── IndexedDB helpers ─────────────────────────────────────────────────────────
+
+function openDB() {
+  return new Promise((resolve, reject) => {
+    const req = indexedDB.open(DB_NAME, DB_VERSION);
+    req.onupgradeneeded = e => {
+      e.target.result.createObjectStore(STORE, { keyPath: 'path' });
+    };
+    req.onsuccess  = e => resolve(e.target.result);
+    req.onerror    = e => reject(e.target.error);
+  });
+}
+
+async function idbGet(path) {
+  const db = await openDB();
+  return new Promise((resolve, reject) => {
+    const req = db.transaction(STORE, 'readonly').objectStore(STORE).get(path);
+    req.onsuccess = e => resolve(e.target.result?.data ?? null);
+    req.onerror   = e => reject(e.target.error);
+  });
+}
+
+// ── Fetch intercept ───────────────────────────────────────────────────────────
+
+self.addEventListener('install',  () => self.skipWaiting());
+self.addEventListener('activate', e  => e.waitUntil(self.clients.claim()));
+
+self.addEventListener('fetch', event => {
+  const url = new URL(event.request.url);
+
+  if (url.pathname === '/data/index.json') {
+    event.respondWith(handleIndex(event.request));
+    return;
+  }
+
+  if (url.pathname.startsWith('/data/activities/')) {
+    event.respondWith(handleActivity(url.pathname, event.request));
+    return;
+  }
+});
+
+// Merge local summaries into the server/static index.json
+async function handleIndex(request) {
+  try {
+    const localSummaries = (await idbGet('/data/local-index')) ?? [];
+
+    if (localSummaries.length === 0) {
+      return fetch(request);  // nothing local — pass straight through
+    }
+
+    // Fetch the bundled static index (may fail if offline with no prior cache)
+    let remoteMeta = {};
+    let remoteActivities = [];
+    try {
+      const r   = await fetch(request);
+      const raw = await r.json();
+      remoteActivities = raw.activities ?? [];
+      const { activities: _a, ...rest } = raw;
+      remoteMeta = rest;
+    } catch (_) {}
+
+    // Local overrides remote for same ID; new local entries appended
+    const merged = new Map();
+    for (const a of remoteActivities) merged.set(a.id, a);
+    for (const a of localSummaries)   merged.set(a.id, a);
+
+    const sorted = [...merged.values()].sort(
+      (a, b) => (b.started_at ?? '').localeCompare(a.started_at ?? '')
+    );
+
+    return jsonResponse({ ...remoteMeta, activities: sorted });
+  } catch (_) {
+    return fetch(request);
+  }
+}
+
+// Serve an individual activity file from IDB if present
+async function handleActivity(path, request) {
+  try {
+    const local = await idbGet(path);
+    if (local !== null) return jsonResponse(local);
+  } catch (_) {}
+  return fetch(request);
+}
+
+function jsonResponse(data) {
+  return new Response(JSON.stringify(data), {
+    headers: { 'Content-Type': 'application/json', 'Cache-Control': 'no-store' },
+  });
+}
diff --git a/site/src/components/ActivityDetail.svelte b/site/src/components/ActivityDetail.svelte
index e5f7014..bf16b09 100644
--- a/site/src/components/ActivityDetail.svelte
+++ b/site/src/components/ActivityDetail.svelte
@@ -7,6 +7,7 @@
   import ActivityMap from './ActivityMap.svelte';
   import ActivityCharts from './ActivityCharts.svelte';
   import EditDrawer from './EditDrawer.svelte';
+  import { loadActivity } from '../lib/dataloader';
 
   export let activity: ActivitySummary;
   export let base: string = '/';
@@ -28,9 +29,8 @@
   onMount(async () => {
     if (!activity.detail_url) return;
     try {
-      const res = await fetch(`${base}data/${activity.detail_url}`);
-      if (!res.ok) throw new Error(`HTTP ${res.status}`);
-      detail = await res.json();
+      detail = await loadActivity(activity.id, activity.detail_url, base);
+      if (!detail) throw new Error('Activity not found');
     } catch (e: any) {
       error = e.message;
     }
diff --git a/site/src/components/ActivityFeed.svelte b/site/src/components/ActivityFeed.svelte
index 7390af8..a0e5240 100644
--- a/site/src/components/ActivityFeed.svelte
+++ b/site/src/components/ActivityFeed.svelte
@@ -2,6 +2,7 @@
   import { onMount } from 'svelte';
   import type { ActivitySummary, BASIndex, Sport } from '../lib/types';
   import { formatDistance, formatDuration, formatElevation, formatDate, sportIcon, sportColor, sportLabel } from '../lib/format';
+  import { loadIndex } from '../lib/dataloader';
 
   /** Render preview_coords as an SVG polyline path string. */
   function trackPath(coords: [number, number][] | null, w: number, h: number): string {
@@ -53,9 +54,7 @@
     sport = (new URLSearchParams(window.location.search).get('sport') as Sport | 'all') ?? 'all';
     mounted = true;
     try {
-      const res = await fetch(`${import.meta.env.BASE_URL}data/index.json`);
-      if (!res.ok) throw new Error(`HTTP ${res.status}`);
-      const index: BASIndex = await res.json();
+      const index = await loadIndex(import.meta.env.BASE_URL);
       all = index.activities.filter(a => a.privacy !== 'private');
     } catch (e: any) {
       error = e.message;
diff --git a/site/src/components/AthleteView.svelte b/site/src/components/AthleteView.svelte
index d7b0b36..102e5e1 100644
--- a/site/src/components/AthleteView.svelte
+++ b/site/src/components/AthleteView.svelte
@@ -4,6 +4,7 @@
   import MmpChart from './MmpChart.svelte';
   import RecordsView from './RecordsView.svelte';
   import AthleteDrawer from './AthleteDrawer.svelte';
+  import { loadIndex, loadAthlete } from '../lib/dataloader';
 
   export let base: string = '/';
 
@@ -32,13 +33,12 @@
     activeTab = TABS.includes(rawTab as Tab) ? (rawTab as Tab) : 'power';
     mounted = true;
     try {
-      const [athleteRes, indexRes] = await Promise.all([
-        fetch(`${import.meta.env.BASE_URL}data/athlete.json`),
-        fetch(`${import.meta.env.BASE_URL}data/index.json`),
+      const [athleteData, index] = await Promise.all([
+        loadAthlete(import.meta.env.BASE_URL),
+        loadIndex(import.meta.env.BASE_URL),
       ]);
-      if (!athleteRes.ok) throw new Error('athlete.json not found — run bincio extract first');
-      athlete = await athleteRes.json();
-      const index: BASIndex = await indexRes.json();
+      if (!athleteData) throw new Error('athlete.json not found — run bincio extract first');
+      athlete = athleteData;
       activities = index.activities.filter(a => a.mmp && a.privacy !== 'private');
     } catch (e: any) {
       error = e.message;
diff --git a/site/src/components/StatsView.svelte b/site/src/components/StatsView.svelte
index d1bee17..be226a0 100644
--- a/site/src/components/StatsView.svelte
+++ b/site/src/components/StatsView.svelte
@@ -2,6 +2,7 @@
   import { onMount } from 'svelte';
   import type { ActivitySummary, BASIndex, Sport } from '../lib/types';
   import { formatDistance, formatDuration, sportIcon, sportColor, sportLabel } from '../lib/format';
+  import { loadIndex } from '../lib/dataloader';
 
   const PAGE_YEARS = 4;
 
@@ -30,9 +31,7 @@
     page  = parseInt(params.get('page') ?? '0', 10) || 0;
     mounted = true;
     try {
-      const res = await fetch(`${import.meta.env.BASE_URL}data/index.json`);
-      if (!res.ok) throw new Error(`HTTP ${res.status}`);
-      const index: BASIndex = await res.json();
+      const index = await loadIndex(import.meta.env.BASE_URL);
       all = index.activities.filter(a => a.privacy !== 'private' && a.distance_m);
     } catch (e: any) {
       error = e.message;
diff --git a/site/src/layouts/Base.astro b/site/src/layouts/Base.astro
index 346e2f4..5d78e83 100644
--- a/site/src/layouts/Base.astro
+++ b/site/src/layouts/Base.astro
@@ -208,6 +208,13 @@ const baseUrl = import.meta.env.BASE_URL ?? '/';
     <slot />
   </main>
 
+  <script>
+    // Register service worker for local activity storage (offline support)
+    if ('serviceWorker' in navigator) {
+      navigator.serviceWorker.register('/sw.js').catch(() => {});
+    }
+  </script>
+
   <script>
     const btn = document.getElementById('theme-toggle') as HTMLButtonElement;
 
diff --git a/site/src/lib/dataloader.ts b/site/src/lib/dataloader.ts
new file mode 100644
index 0000000..d3fc10a
--- /dev/null
+++ b/site/src/lib/dataloader.ts
@@ -0,0 +1,123 @@
+/**
+ * Data access abstraction layer.
+ *
+ * All Svelte components load BAS data through these functions instead of
+ * calling fetch() directly. Each function merges server/bundled data with
+ * any activities stored locally in IndexedDB (via localstore.ts), so the
+ * app works the same whether it is connected to a cloud instance, running
+ * offline, or somewhere in between.
+ *
+ * Design notes:
+ * - Server fetch and IDB read run concurrently (Promise.allSettled).
+ * - If the server is unreachable, local-only data is returned.
+ * - If IDB is empty, pure server data is returned — zero overhead.
+ * - Local activities override server ones with the same ID (local is authoritative
+ *   for anything the user recorded or converted on this device).
+ */
+
+import type { ActivityDetail, ActivitySummary, BASIndex } from './types';
+import { listLocalActivities } from './localstore';
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+async function fetchJSON<T>(url: string): Promise<T> {
+  const r = await fetch(url);
+  if (!r.ok) throw new Error(`HTTP ${r.status}`);
+  return r.json() as Promise<T>;
+}
+
+async function idbGetActivity(id: string): Promise<ActivityDetail | null> {
+  // Inline IDB read — avoids importing openDB into every caller
+  return new Promise(resolve => {
+    try {
+      const req = indexedDB.open('bincio', 1);
+      req.onsuccess = e => {
+        const db: IDBDatabase = (e.target as IDBOpenDBRequest).result;
+        const tx = db.transaction('files', 'readonly');
+        const get = tx.objectStore('files').get(`/data/activities/${id}.json`);
+        get.onsuccess = ge => resolve((ge.target as IDBRequest).result?.data ?? null);
+        get.onerror   = () => resolve(null);
+      };
+      req.onerror = () => resolve(null);
+    } catch {
+      resolve(null);
+    }
+  });
+}
+
+function emptyIndex(): BASIndex {
+  return {
+    bas_version: '1.0',
+    owner: { handle: 'unknown', display_name: '' },
+    generated_at: '',
+    shards: [],
+    activities: [],
+  };
+}
+
+// ── Public API ────────────────────────────────────────────────────────────────
+
+/**
+ * Load the activity index, merging the server's copy with any locally-stored
+ * activities. Local entries override server entries with the same ID.
+ */
+export async function loadIndex(baseUrl: string): Promise<BASIndex> {
+  const [serverResult, localResult] = await Promise.allSettled([
+    fetchJSON<BASIndex>(`${baseUrl}data/index.json`),
+    listLocalActivities(),
+  ]);
+
+  const server = serverResult.status === 'fulfilled' ? serverResult.value : null;
+  const local  = localResult.status  === 'fulfilled' ? localResult.value  : [];
+
+  if (local.length === 0) return server ?? emptyIndex();
+  if (!server)            return { ...emptyIndex(), activities: local as ActivitySummary[] };
+
+  // Local overrides server for the same ID; new local entries are appended
+  const merged = new Map<string, ActivitySummary>();
+  for (const a of server.activities ?? []) merged.set(a.id, a);
+  for (const a of local as ActivitySummary[]) merged.set(a.id, a);
+
+  return {
+    ...server,
+    activities: [...merged.values()].sort(
+      (a, b) => (b.started_at ?? '').localeCompare(a.started_at ?? ''),
+    ),
+  };
+}
+
+/**
+ * Load a single activity detail, checking IndexedDB first so locally-converted
+ * activities are available offline.
+ *
+ * @param id        Activity ID (used for the IDB lookup)
+ * @param detailUrl Relative path from the BAS index (e.g. "activities/id.json")
+ * @param baseUrl   Site base URL
+ */
+export async function loadActivity(
+  id:        string,
+  detailUrl: string,
+  baseUrl:   string,
+): Promise<ActivityDetail | null> {
+  // IDB first — instant and works offline
+  const cached = await idbGetActivity(id);
+  if (cached) return cached;
+
+  try {
+    return await fetchJSON<ActivityDetail>(`${baseUrl}data/${detailUrl}`);
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Load athlete profile. Athlete data is not stored locally yet, so this is
+ * always a network fetch with a graceful null on failure.
+ */
+export async function loadAthlete(baseUrl: string): Promise<Record<string, unknown> | null> {
+  try {
+    return await fetchJSON(`${baseUrl}data/athlete.json`);
+  } catch {
+    return null;
+  }
+}
diff --git a/site/src/lib/localstore.ts b/site/src/lib/localstore.ts
new file mode 100644
index 0000000..065439a
--- /dev/null
+++ b/site/src/lib/localstore.ts
@@ -0,0 +1,89 @@
+/**
+ * IndexedDB helper for local activity storage.
+ *
+ * Activities converted on-device are written here. The service worker (sw.js)
+ * reads from the same database and merges local activities into the feed.
+ */
+
+const DB_NAME    = 'bincio';
+const DB_VERSION = 1;
+const STORE      = 'files';
+
+function openDB(): Promise<IDBDatabase> {
+  return new Promise((resolve, reject) => {
+    const req = indexedDB.open(DB_NAME, DB_VERSION);
+    req.onupgradeneeded = e =>
+      (e.target as IDBOpenDBRequest).result.createObjectStore(STORE, { keyPath: 'path' });
+    req.onsuccess = e => resolve((e.target as IDBOpenDBRequest).result);
+    req.onerror   = e => reject((e.target as IDBOpenDBRequest).error);
+  });
+}
+
+async function idbPut(path: string, data: unknown): Promise<void> {
+  const db = await openDB();
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(STORE, 'readwrite');
+    tx.objectStore(STORE).put({ path, data });
+    tx.oncomplete = () => resolve();
+    tx.onerror    = e => reject((e.target as IDBTransaction).error);
+  });
+}
+
+async function idbGet<T>(path: string): Promise<T | null> {
+  const db = await openDB();
+  return new Promise((resolve, reject) => {
+    const req = db.transaction(STORE, 'readonly').objectStore(STORE).get(path);
+    req.onsuccess = e => resolve((e.target as IDBRequest).result?.data ?? null);
+    req.onerror   = e => reject((e.target as IDBRequest).error);
+  });
+}
+
+// ── Public API ────────────────────────────────────────────────────────────────
+
+/** Save a converted activity to IndexedDB and update the local summary index. */
+export async function saveActivityLocally(
+  detail:  Record<string, unknown>,
+  geojson: Record<string, unknown> | null,
+): Promise<void> {
+  const id = detail.id as string;
+
+  await idbPut(`/data/activities/${id}.json`, detail);
+  if (geojson) {
+    await idbPut(`/data/activities/${id}.geojson`, geojson);
+  }
+
+  // Maintain a flat list of local summaries (read by the service worker)
+  const existing = (await idbGet<ActivitySummary[]>('/data/local-index')) ?? [];
+  const summary  = toSummary(detail);
+  const idx = existing.findIndex(a => a.id === id);
+  if (idx >= 0) existing[idx] = summary; else existing.push(summary);
+  await idbPut('/data/local-index', existing);
+}
+
+/** Return all locally-stored activity summaries. */
+export async function listLocalActivities(): Promise<ActivitySummary[]> {
+  return (await idbGet<ActivitySummary[]>('/data/local-index')) ?? [];
+}
+
+/** Return true if at least one activity is stored locally. */
+export async function hasLocalActivities(): Promise<boolean> {
+  const list = await listLocalActivities();
+  return list.length > 0;
+}
+
+// ── Helpers ───────────────────────────────────────────────────────────────────
+
+type ActivitySummary = Record<string, unknown>;
+
+const SUMMARY_KEYS = [
+  'id', 'title', 'sport', 'sub_sport', 'started_at', 'distance_m',
+  'duration_s', 'moving_time_s', 'elevation_gain_m', 'avg_speed_kmh',
+  'avg_hr_bpm', 'avg_cadence_rpm', 'avg_power_w', 'privacy',
+  'detail_url', 'track_url', 'preview_coords',
+] as const;
+
+function toSummary(detail: Record<string, unknown>): ActivitySummary {
+  return Object.fromEntries(
+    SUMMARY_KEYS.filter(k => k in detail).map(k => [k, detail[k]])
+  );
+}
diff --git a/site/src/pages/convert/index.astro b/site/src/pages/convert/index.astro
index bfcf490..7c5dc13 100644
--- a/site/src/pages/convert/index.astro
+++ b/site/src/pages/convert/index.astro
@@ -57,6 +57,9 @@ const baseUrl = import.meta.env.BASE_URL ?? '/';
         </div>
 
         <div class="flex flex-col gap-2">
+          <button id="btn-save-local" class="w-full py-2 px-4 rounded-lg text-sm font-medium text-white transition-colors" style="background:#3b82f6">
+            📱 Save to this device
+          </button>
           <button id="btn-download-json" class="w-full py-2 px-4 rounded-lg text-sm font-medium bg-zinc-700 hover:bg-zinc-600 text-white transition-colors">
             ⬇ Download BAS JSON
           </button>
@@ -64,8 +67,8 @@ const baseUrl = import.meta.env.BASE_URL ?? '/';
             ⬇ Download GeoJSON track
           </button>
           {editUrl && (
-            <button id="btn-save" class="w-full py-2 px-4 rounded-lg text-sm font-medium text-white transition-colors" style="background:#3b82f6">
-              ☁ Save to my bincio
+            <button id="btn-save" class="w-full py-2 px-4 rounded-lg text-sm font-medium bg-zinc-800 hover:bg-zinc-700 text-white transition-colors border border-zinc-700">
+              ☁ Save to cloud instance
             </button>
           )}
         </div>
@@ -85,7 +88,16 @@ const baseUrl = import.meta.env.BASE_URL ?? '/';
   </div>
 </Base>
 
-<script define:vars={{ editUrl, baseUrl }}>
+<div id="conv-config" data-edit-url={editUrl} data-base-url={baseUrl} style="display:none"></div>
+
+<script>
+  import { saveActivityLocally } from '../../lib/localstore';
+
+  // ── Config ──────────────────────────────────────────────────────────────────
+  const _cfg     = document.getElementById('conv-config') as HTMLElement;
+  const editUrl  = _cfg.dataset.editUrl  ?? '';
+  const baseUrl  = _cfg.dataset.baseUrl  ?? '/';
+
   // ── DOM refs ────────────────────────────────────────────────────────────────
   const stepPick    = document.getElementById('step-pick');
   const stepLoading = document.getElementById('step-loading');
@@ -168,17 +180,17 @@ const baseUrl = import.meta.env.BASE_URL ?? '/';
     setProgress(20, 'Initialising Python…');
     pyodide = await window.loadPyodide({ indexURL: 'https://cdn.jsdelivr.net/pyodide/v0.26.4/full/' });
 
-    setProgress(40, 'Loading packages (lxml, pyyaml)…');
-    await pyodide.loadPackage(['lxml', 'pyyaml']);
+    setProgress(40, 'Loading packages (lxml, pyyaml, micropip)…');
+    await pyodide.loadPackage(['lxml', 'pyyaml', 'micropip']);
 
     setProgress(60, 'Installing fitdecode, gpxpy, rdp…');
     await pyodide.runPythonAsync(`
 import micropip
-await micropip.install(['fitdecode', 'gpxpy', 'rdp'])
+await micropip.install(['fitdecode', 'gpxpy'])
 `);
 
     setProgress(80, 'Loading bincio extract pipeline…');
-    const wheelUrl = new URL('/bincio.whl', window.location.origin).href;
+    const wheelUrl = new URL('/bincio-0.1.0-py3-none-any.whl', window.location.origin).href;
     await pyodide.runPythonAsync(`
 import micropip
 await micropip.install('${wheelUrl}', deps=False)
@@ -276,6 +288,27 @@ json.dumps({'id': act_id, 'detail': detail, 'geojson': geojson})
     showStep('error');
   }
 
+  // ── Save locally (IndexedDB → service worker serves it in the feed) ─────────
+  document.getElementById('btn-save-local').addEventListener('click', async () => {
+    if (!lastResult) return;
+    const btn = document.getElementById('btn-save-local');
+    btn.disabled = true;
+    btn.textContent = 'Saving…';
+    saveStatus.style.color = 'var(--text-4)';
+    try {
+      await saveActivityLocally(lastResult.detail, lastResult.geojson ?? null);
+      btn.textContent = '✓ Saved to device';
+      btn.style.background = '#16a34a';
+      saveStatus.textContent = 'Activity saved. Visit the feed to see it.';
+      saveStatus.style.color = '#4ade80';
+    } catch (e) {
+      btn.disabled = false;
+      btn.textContent = '📱 Save to this device';
+      saveStatus.textContent = 'Save failed: ' + e.message;
+      saveStatus.style.color = '#f87171';
+    }
+  });
+
   // ── Downloads ───────────────────────────────────────────────────────────────
   function downloadJson(data, filename) {
     const blob = new Blob([JSON.stringify(data, null, 2)], { type: 'application/json' });