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/setup.ts

@@ -1,4 +1,5 @@
 import '@testing-library/jest-dom/vitest';
+import 'fake-indexeddb/auto';
 import { cleanup } from '@testing-library/react';
 import { afterAll, afterEach, beforeAll, vi } from 'vitest';
 import { server } from './helpers/msw/server';
@@ -9,6 +10,7 @@ vi.mock('../src/api/websocket', () => ({
   disconnect: vi.fn(),
   getSocketId: vi.fn(() => null),
   setRefetchCallback: vi.fn(),
+  setPreReconnectHook: vi.fn(),
 }));
 
 // MSW lifecycle