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/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()}>
-        <Text style={styles.backText}>← Back</Text>
-      </Pressable>
+      <View style={styles.topBar}>
+        <Pressable style={styles.backButton} onPress={() => router.back()}>
+          <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 },