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

server/src/routes/trips.ts

@@ -29,6 +29,13 @@ import {
   ValidationError,
   TRIP_SELECT,
 } from '../services/tripService';
+import { listDays } from '../services/dayService';
+import { listPlaces } from '../services/placeService';
+import { listItems as listPackingItems } from '../services/packingService';
+import { listItems as listTodoItems } from '../services/todoService';
+import { listBudgetItems } from '../services/budgetService';
+import { listReservations } from '../services/reservationService';
+import { listFiles } from '../services/fileService';
 
 const router = express.Router();
 
@@ -294,6 +301,36 @@ router.delete('/:id/members/:userId', authenticate, (req: Request, res: Response
   res.json({ success: true });
 });
 
+// ── Offline bundle ────────────────────────────────────────────────────────
+// Returns all trip sub-collections in a single request for offline caching.
+
+router.get('/:id/bundle', authenticate, (req: Request, res: Response) => {
+  const authReq = req as AuthRequest;
+  const tripId = req.params.id;
+
+  const trip = getTrip(tripId, authReq.user.id);
+  if (!trip) return res.status(404).json({ error: 'Trip not found' });
+
+  const { days } = listDays(tripId);
+  const places = listPlaces(String(tripId), {});
+  const packingItems = listPackingItems(tripId);
+  const todoItems = listTodoItems(tripId);
+  const budgetItems = listBudgetItems(tripId);
+  const reservations = listReservations(tripId);
+  const files = listFiles(tripId, false);
+
+  res.json({
+    trip,
+    days,
+    places,
+    packingItems,
+    todoItems,
+    budgetItems,
+    reservations,
+    files,
+  });
+});
+
 // ── ICS calendar export ───────────────────────────────────────────────────
 
 router.get('/:id/export.ics', authenticate, (req: Request, res: Response) => {