feat(pwa): implement real offline mode with IndexedDB sync

Add genuine offline read/write capability for trips:

- Dexie IndexedDB schema (trips, places, packing, todo, budget,
  reservations, files, mutationQueue, syncMeta, blobCache)
- Repo layer for all domains: offline reads from Dexie, writes
  optimistically to Dexie and enqueue mutations for later replay
- Mutation queue with UUID idempotency keys (X-Idempotency-Key),
  FIFO flush, temp-ID reconciliation on 2xx, fail-and-continue on 4xx
- Trip sync manager: caches all trips with end_date >= today or null,
  auto-evicts 7d after end_date, fetches bundle endpoint in one request
- Map tile prefetcher: bbox from place coords, zooms 10-16, 50MB cap,
  warms SW cache via fetch
- Sync triggers: network online → flush + syncAll; WS reconnect →
  flush only (rate-limiter safe); visibilitychange/30s → flush only
- WS remoteEventHandler writes through to Dexie on every event
- Server idempotency middleware + idempotency_keys table (migration 100,
  24h TTL nightly cleanup)
- GET /api/trips/:id/bundle endpoint for efficient single-request sync
- OfflineBanner component: amber (offline) / blue (syncing) / hidden
- OfflineTab in Settings: cached trip list, re-sync and clear actions
- usePendingMutations hook for per-item pending indicators

Closes #505 #541

client/tests/helpers/msw/handlers/trips.ts

@@ -1,5 +1,5 @@
 import { http, HttpResponse } from 'msw';
-import { buildTrip, buildDay, buildUser } from '../../factories';
+import { buildTrip, buildDay, buildUser, buildPlace, buildPackingItem, buildTodoItem, buildBudgetItem, buildReservation, buildTripFile } from '../../factories';
 
 export const tripsHandlers = [
   // List all trips (active or archived)
@@ -47,6 +47,22 @@ export const tripsHandlers = [
     return HttpResponse.json({ accommodations: [] });
   }),
 
+  http.get('/api/trips/:id/bundle', ({ params }) => {
+    const tripId = Number(params.id);
+    const trip = buildTrip({ id: tripId });
+    const day = buildDay({ trip_id: tripId, assignments: [], notes_items: [] });
+    return HttpResponse.json({
+      trip,
+      days: [day],
+      places: [buildPlace({ trip_id: tripId })],
+      packingItems: [buildPackingItem({ trip_id: tripId })],
+      todoItems: [buildTodoItem({ trip_id: tripId })],
+      budgetItems: [buildBudgetItem({ trip_id: tripId })],
+      reservations: [buildReservation({ trip_id: tripId })],
+      files: [buildTripFile({ trip_id: tripId })],
+    });
+  }),
+
   http.delete('/api/trips/:id', () => {
     return HttpResponse.json({ success: true });
   }),