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/package.json

@@ -17,6 +17,7 @@
   "dependencies": {
     "@react-pdf/renderer": "^4.3.2",
     "axios": "^1.6.7",
+    "dexie": "^4.4.2",
     "leaflet": "^1.9.4",
     "lucide-react": "^0.344.0",
     "marked": "^18.0.0",
@@ -44,6 +45,7 @@
     "@vitejs/plugin-react": "^4.2.1",
     "@vitest/coverage-v8": "^3.2.4",
     "autoprefixer": "^10.4.18",
+    "fake-indexeddb": "^6.2.5",
     "jsdom": "^29.0.1",
     "msw": "^2.13.0",
     "postcss": "^8.4.35",