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/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(