perf: year-shard index.json to cut initial load from MBs to ~1 year

merge_all/_merged/index.json is now a shard manifest; activities are
split into index-{year}.json files. The feed loads only the most-recent
year on first paint (~200 activities instead of all of them). Older
years are fetched lazily when the user clicks "Load older activities".

Also strips best_efforts / best_climb_m / source from shard files —
these fields are aggregation inputs only, never read by the feed UI.

site/src/components/ActivityFeed.svelte

@@ -2,7 +2,7 @@
   import { onMount } from 'svelte';
   import type { ActivitySummary, BASIndex, Sport } from '../lib/types';
   import { formatDistance, formatDuration, formatElevation, formatDate, isUnlisted, sportIcon, sportColor, sportLabel } from '../lib/format';
-  import { loadIndex } from '../lib/dataloader';
+  import { loadIndexPaged, loadShardActivities } from '../lib/dataloader';
 
   /** Render preview_coords as an SVG polyline path string. */
   function trackPath(coords: [number, number][] | null, w: number, h: number): string {
@@ -41,8 +41,10 @@
   let sport: Sport | 'all' = 'all';
   let shown = PAGE_SIZE;
   let loading = true;
+  let loadingMore = false;
   let error = '';
   let mounted = false;
+  let pendingShards: string[] = [];
   /** Logged-in handle — resolved async via bincio:me event. */
   let me: string = '';
 
@@ -58,7 +60,33 @@
   });
   $: filtered = sport === 'all' ? withPrivacy : withPrivacy.filter(a => a.sport === sport);
   $: visible = filtered.slice(0, shown);
-  $: hasMore = shown < filtered.length;
+  $: canShowMore = shown < filtered.length;
+  $: hasMore = canShowMore || pendingShards.length > 0;
+
+  async function loadMore() {
+    if (canShowMore) {
+      shown += PAGE_SIZE;
+      return;
+    }
+    if (!pendingShards.length) return;
+    loadingMore = true;
+    const url = pendingShards[0];
+    pendingShards = pendingShards.slice(1);
+    try {
+      const fresh = await loadShardActivities(url);
+      // Merge avoiding duplicates (IDB activities may already be present)
+      const existing = new Map(all.map(a => [a.id, a]));
+      for (const a of fresh) if (!existing.has(a.id)) existing.set(a.id, a);
+      all = [...existing.values()].sort((a, b) =>
+        (b.started_at ?? '').localeCompare(a.started_at ?? ''),
+      );
+      shown += PAGE_SIZE;
+    } catch {
+      // shard load failed — don't block the user
+    } finally {
+      loadingMore = false;
+    }
+  }
 
   $: if (sport) shown = PAGE_SIZE; // reset pagination on filter change
 
@@ -84,12 +112,9 @@
       const indexUrl = profileIndexUrl
         ? `${base}data/${profileIndexUrl}`
         : `${base}data/index.json`;
-      const index = await loadIndex(base, indexUrl);
+      const { index, pendingShards: pending } = await loadIndexPaged(base, indexUrl);
+      pendingShards = pending;
       let activities = index.activities;
-      // filterHandle only applies when loading the root manifest (multi-user feed).
-      // When profileIndexUrl is set we already loaded the right user's shard directly —
-      // activities from a direct shard fetch have no handle tag, so the filter would
-      // remove everything.
       if (filterHandle && !profileIndexUrl) {
         activities = activities.filter(a => (a as any).handle === filterHandle);
       }
@@ -230,10 +255,17 @@
   {#if hasMore}
     <div class="text-center mt-8">
       <button
-        class="px-6 py-2 rounded-full border border-zinc-700 text-zinc-300 hover:border-zinc-500 hover:text-white transition-colors text-sm"
-        on:click={() => shown += PAGE_SIZE}
+        class="px-6 py-2 rounded-full border border-zinc-700 text-zinc-300 hover:border-zinc-500 hover:text-white disabled:opacity-40 transition-colors text-sm"
+        disabled={loadingMore}
+        on:click={loadMore}
       >
-        Load more ({filtered.length - shown} remaining)
+        {#if loadingMore}
+          Loading…
+        {:else if canShowMore}
+          Load more ({filtered.length - shown} remaining)
+        {:else}
+          Load older activities ({pendingShards.length} more {pendingShards.length === 1 ? 'year' : 'years'})
+        {/if}
       </button>
     </div>
   {/if}