feat: add delete button for local activities (single and bulk)

- Detail screen: Delete button (top-right, red) with confirmation alert;
  deletes SQLite row and original file via expo-file-system
- Feed screen: long-press card to enter select mode; checkbox + blue
  border on selected cards; bottom action bar with bulk Delete N button;
  header switches to show count + Cancel
- db/queries: deleteActivity (returns original_path) and deleteActivities
  (bulk, returns all original paths)

mobile/app/(tabs)/index.tsx

@@ -1,8 +1,9 @@
+import * as FileSystem from 'expo-file-system';
 import { useSQLiteContext } from 'expo-sqlite';
 import { useRouter } from 'expo-router';
 import { useCallback, useState } from 'react';
-import { FlatList, Pressable, RefreshControl, StyleSheet, Text, View } from 'react-native';
+import { Alert, FlatList, Pressable, RefreshControl, StyleSheet, Text, View } from 'react-native';
-import { useActivities, type ActivitySummary } from '@/db/queries';
+import { deleteActivities, useActivities, type ActivitySummary } from '@/db/queries';
 import { syncFeed } from '@/db/sync';
 
 export default function FeedScreen() {
@@ -10,6 +11,8 @@ export default function FeedScreen() {
   const activities = useActivities();
   const [syncing, setSyncing] = useState(false);
   const [syncMsg, setSyncMsg] = useState<string | null>(null);
+  const [selected, setSelected] = useState<Set<string>>(new Set());
+  const selecting = selected.size > 0;
 
   const doSync = useCallback(async () => {
     setSyncing(true);
@@ -32,16 +35,64 @@ export default function FeedScreen() {
     setTimeout(() => setSyncMsg(null), 3500);
   }, [db]);
 
+  function toggleSelect(id: string) {
+    setSelected(prev => {
+      const next = new Set(prev);
+      if (next.has(id)) next.delete(id); else next.add(id);
+      return next;
+    });
+  }
+
+  function cancelSelect() {
+    setSelected(new Set());
+  }
+
+  function confirmDeleteSelected() {
+    const count = selected.size;
+    Alert.alert(
+      `Delete ${count} activit${count === 1 ? 'y' : 'ies'}`,
+      'These activities will be permanently removed from your device.',
+      [
+        { text: 'Cancel', style: 'cancel' },
+        {
+          text: 'Delete',
+          style: 'destructive',
+          onPress: async () => {
+            const ids = Array.from(selected);
+            const paths = await deleteActivities(db, ids);
+            setSelected(new Set());
+            for (const p of paths) {
+              if (p) {
+                try { await FileSystem.deleteAsync(p, { idempotent: true }); } catch {}
+              }
+            }
+          },
+        },
+      ],
+    );
+  }
+
   return (
     <View style={styles.container}>
       <View style={styles.headerRow}>
-        <Text style={styles.header}>Feed</Text>
+        {selecting ? (
-        <Pressable
+          <>
-          style={[styles.syncButton, syncing && styles.syncButtonDisabled]}
+            <Text style={styles.header}>{selected.size} selected</Text>
-          onPress={syncing ? undefined : doSync}
+            <Pressable style={styles.cancelButton} onPress={cancelSelect}>
-        >
+              <Text style={styles.cancelText}>Cancel</Text>
-          <Text style={styles.syncText}>{syncing ? 'Syncing…' : '↓ Sync'}</Text>
+            </Pressable>
-        </Pressable>
+          </>
+        ) : (
+          <>
+            <Text style={styles.header}>Feed</Text>
+            <Pressable
+              style={[styles.syncButton, syncing && styles.syncButtonDisabled]}
+              onPress={syncing ? undefined : doSync}
+            >
+              <Text style={styles.syncText}>{syncing ? 'Syncing…' : '↓ Sync'}</Text>
+            </Pressable>
+          </>
+        )}
       </View>
 
       {syncMsg && (
@@ -60,7 +111,15 @@ export default function FeedScreen() {
         <FlatList
           data={activities}
           keyExtractor={(a) => a.id}
-          renderItem={({ item }) => <ActivityCard activity={item} />}
+          renderItem={({ item }) => (
+            <ActivityCard
+              activity={item}
+              selecting={selecting}
+              checked={selected.has(item.id)}
+              onToggleSelect={() => toggleSelect(item.id)}
+              onLongPress={() => toggleSelect(item.id)}
+            />
+          )}
           contentContainerStyle={styles.list}
           refreshControl={
             <RefreshControl
@@ -71,11 +130,31 @@ export default function FeedScreen() {
           }
         />
       )}
+
+      {selecting && (
+        <View style={styles.actionBar}>
+          <Pressable style={styles.deleteBarButton} onPress={confirmDeleteSelected}>
+            <Text style={styles.deleteBarText}>Delete {selected.size}</Text>
+          </Pressable>
+        </View>
+      )}
     </View>
   );
 }
 
-function ActivityCard({ activity }: { activity: ActivitySummary }) {
+function ActivityCard({
+  activity,
+  selecting,
+  checked,
+  onToggleSelect,
+  onLongPress,
+}: {
+  activity: ActivitySummary;
+  selecting: boolean;
+  checked: boolean;
+  onToggleSelect: () => void;
+  onLongPress: () => void;
+}) {
   const router = useRouter();
   const km   = activity.distance_m != null ? (activity.distance_m / 1000).toFixed(1) : null;
   const elev = activity.elevation_gain_m != null ? Math.round(activity.elevation_gain_m) : null;
@@ -83,13 +162,29 @@ function ActivityCard({ activity }: { activity: ActivitySummary }) {
     day: 'numeric', month: 'short', year: 'numeric',
   });
 
+  function handlePress() {
+    if (selecting) {
+      onToggleSelect();
+    } else {
+      router.push(`/activity/${activity.id}`);
+    }
+  }
+
   return (
     <Pressable
-      style={styles.card}
+      style={[styles.card, checked && styles.cardSelected]}
-      onPress={() => router.push(`/activity/${activity.id}`)}
+      onPress={handlePress}
+      onLongPress={onLongPress}
     >
       <View style={styles.cardTop}>
-        <Text style={styles.sportIcon}>{sportIcon(activity.sport)}</Text>
+        <View style={styles.cardLeft}>
+          {selecting && (
+            <View style={[styles.checkbox, checked && styles.checkboxChecked]}>
+              {checked && <Text style={styles.checkmark}>✓</Text>}
+            </View>
+          )}
+          <Text style={styles.sportIcon}>{sportIcon(activity.sport)}</Text>
+        </View>
         <View style={styles.cardMeta}>
           <Text style={styles.cardDate}>{date}</Text>
           {activity.origin === 'remote'
@@ -136,16 +231,23 @@ const styles = StyleSheet.create({
   },
   syncButtonDisabled: { opacity: 0.5 },
   syncText: { color: '#60a5fa', fontSize: 13, fontWeight: '600' },
+  cancelButton: {
+    backgroundColor: '#27272a', borderRadius: 8,
+    paddingHorizontal: 14, paddingVertical: 7,
+  },
+  cancelText: { color: '#a1a1aa', fontSize: 13, fontWeight: '600' },
   syncMsg: {
     color: '#a1a1aa', fontSize: 12, textAlign: 'center',
     paddingHorizontal: 16, paddingBottom: 8,
   },
-  list:  { padding: 16, gap: 12 },
+  list:  { padding: 16, gap: 12, paddingBottom: 80 },
   card: {
     backgroundColor: '#18181b', borderRadius: 12,
     padding: 16, borderWidth: 1, borderColor: '#27272a',
   },
+  cardSelected: { borderColor: '#60a5fa' },
   cardTop:  { flexDirection: 'row', justifyContent: 'space-between', marginBottom: 6 },
+  cardLeft: { flexDirection: 'row', alignItems: 'center', gap: 10 },
   sportIcon: { fontSize: 20 },
   cardMeta: { flexDirection: 'row', alignItems: 'center', gap: 8 },
   cardDate: { color: '#71717a', fontSize: 12 },
@@ -162,10 +264,26 @@ const styles = StyleSheet.create({
   stat:       { flexDirection: 'row', alignItems: 'baseline', gap: 3 },
   statValue:  { color: '#f4f4f5', fontSize: 16, fontWeight: '600' },
   statLabel:  { color: '#71717a', fontSize: 12 },
+  checkbox: {
+    width: 20, height: 20, borderRadius: 4, borderWidth: 1.5,
+    borderColor: '#52525b', alignItems: 'center', justifyContent: 'center',
+  },
+  checkboxChecked: { backgroundColor: '#60a5fa', borderColor: '#60a5fa' },
+  checkmark: { color: '#fff', fontSize: 12, fontWeight: '700' },
   empty: {
     flex: 1, alignItems: 'center', justifyContent: 'center', padding: 32,
   },
   emptyIcon:  { fontSize: 48, marginBottom: 16 },
   emptyTitle: { color: '#f4f4f5', fontSize: 18, fontWeight: '600', marginBottom: 8 },
   emptyBody:  { color: '#71717a', fontSize: 14, textAlign: 'center', lineHeight: 20 },
+  actionBar: {
+    position: 'absolute', bottom: 0, left: 0, right: 0,
+    backgroundColor: '#18181b', borderTopWidth: 1, borderTopColor: '#27272a',
+    paddingHorizontal: 16, paddingVertical: 12, paddingBottom: 28,
+  },
+  deleteBarButton: {
+    backgroundColor: '#7f1d1d', borderRadius: 10,
+    paddingVertical: 14, alignItems: 'center',
+  },
+  deleteBarText: { color: '#fca5a5', fontSize: 15, fontWeight: '700' },
 });

mobile/app/activity/[id].tsx

@@ -1,9 +1,11 @@
 import { Camera, GeoJSONSource, Layer, Map } from '@maplibre/maplibre-react-native';
+import * as FileSystem from 'expo-file-system';
 import { useLocalSearchParams, useRouter } from 'expo-router';
 import { useEffect, useState } from 'react';
-import { ActivityIndicator, Modal, Pressable, ScrollView, StyleSheet, Text, View } from 'react-native';
+import { ActivityIndicator, Alert, Modal, Pressable, ScrollView, StyleSheet, Text, View } from 'react-native';
 import Svg, { Defs, LinearGradient, Path, Stop } from 'react-native-svg';
-import { useActivity, useSetting } from '@/db/queries';
+import { useSQLiteContext } from 'expo-sqlite';
+import { deleteActivity, useActivity, useSetting } from '@/db/queries';
 
 const MAP_STYLE = 'https://basemaps.cartocdn.com/gl/dark-matter-gl-style/style.json';
 
@@ -25,6 +27,7 @@ type Timeseries = {
 export default function ActivityScreen() {
   const { id } = useLocalSearchParams<{ id: string }>();
   const router = useRouter();
+  const db = useSQLiteContext();
   const row = useActivity(id);
   const instanceUrl = useSetting('instance_url')?.replace(/\/$/, '') ?? '';
   const token = useSetting('api_token') ?? '';
@@ -34,6 +37,27 @@ export default function ActivityScreen() {
   const [loadingMap, setLoadingMap] = useState(false);
   const [loadingChart, setLoadingChart] = useState(false);
 
+  async function confirmDelete() {
+    Alert.alert(
+      'Delete activity',
+      'This will permanently remove this activity from your device.',
+      [
+        { text: 'Cancel', style: 'cancel' },
+        {
+          text: 'Delete',
+          style: 'destructive',
+          onPress: async () => {
+            const originalPath = await deleteActivity(db, id);
+            if (originalPath) {
+              try { await FileSystem.deleteAsync(originalPath, { idempotent: true }); } catch {}
+            }
+            router.back();
+          },
+        },
+      ],
+    );
+  }
+
   // instanceUrl and token are in the dep array to avoid a stale-closure bug in
   // release builds: Hermes executes effects sooner and captures empty strings if
   // the deps are omitted.  Guards on geojson/timeseries prevent double-fetching.
@@ -90,9 +114,14 @@ export default function ActivityScreen() {
 
   return (
     <ScrollView style={styles.container} contentContainerStyle={styles.content}>
-      <Pressable style={styles.backButton} onPress={() => router.back()}>
+      <View style={styles.topBar}>
-        <Text style={styles.backText}>← Back</Text>
+        <Pressable style={styles.backButton} onPress={() => router.back()}>
-      </Pressable>
+          <Text style={styles.backText}>← Back</Text>
+        </Pressable>
+        <Pressable style={styles.deleteButton} onPress={confirmDelete}>
+          <Text style={styles.deleteText}>Delete</Text>
+        </Pressable>
+      </View>
 
       <Text style={styles.sport}>{detail.sport ?? 'Activity'}</Text>
       <Text style={styles.title}>{detail.title}</Text>
@@ -367,8 +396,11 @@ const styles = StyleSheet.create({
   content:        { paddingBottom: 40 },
   center:         { flex: 1, alignItems: 'center', justifyContent: 'center', backgroundColor: '#09090b' },
   notFound:       { color: '#71717a', fontSize: 16 },
-  backButton:     { paddingHorizontal: 16, paddingTop: 60, paddingBottom: 12 },
+  topBar:         { flexDirection: 'row', justifyContent: 'space-between', alignItems: 'center', paddingTop: 60, paddingBottom: 12 },
+  backButton:     { paddingHorizontal: 16 },
   backText:       { color: '#60a5fa', fontSize: 15 },
+  deleteButton:   { paddingHorizontal: 16 },
+  deleteText:     { color: '#f87171', fontSize: 15 },
   sport:          { color: '#71717a', fontSize: 12, fontWeight: '600', letterSpacing: 0.8, paddingHorizontal: 16, marginBottom: 4 },
   title:          { color: '#f4f4f5', fontSize: 22, fontWeight: '700', paddingHorizontal: 16, marginBottom: 4 },
   date:           { color: '#71717a', fontSize: 13, paddingHorizontal: 16, marginBottom: 16 },

mobile/db/queries.ts

@@ -110,6 +110,32 @@ export async function deleteRemoteActivities(
   return result.changes;
 }
 
+export async function deleteActivity(
+  db: ReturnType<typeof useSQLiteContext>,
+  id: string,
+): Promise<string | null> {
+  const row = db.getFirstSync<{ original_path: string | null }>(
+    'SELECT original_path FROM activities WHERE id = ?',
+    [id],
+  );
+  await db.runAsync('DELETE FROM activities WHERE id = ?', [id]);
+  return row?.original_path ?? null;
+}
+
+export async function deleteActivities(
+  db: ReturnType<typeof useSQLiteContext>,
+  ids: string[],
+): Promise<Array<string | null>> {
+  if (ids.length === 0) return [];
+  const rows = db.getAllSync<{ original_path: string | null }>(
+    `SELECT original_path FROM activities WHERE id IN (${ids.map(() => '?').join(',')})`,
+    ids,
+  );
+  const placeholders = ids.map(() => '?').join(',');
+  await db.runAsync(`DELETE FROM activities WHERE id IN (${placeholders})`, ids);
+  return rows.map(r => r.original_path ?? null);
+}
+
 // ── Settings ───────────────────────────────────────────────────────────────
 
 export async function getSetting(