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/src/App.tsx

@@ -22,6 +22,8 @@ import { TranslationProvider, useTranslation } from './i18n'
 import { authApi } from './api/client'
 import { usePermissionsStore, PermissionLevel } from './store/permissionsStore'
 import { useInAppNotificationListener } from './hooks/useInAppNotificationListener.ts'
+import { registerSyncTriggers, unregisterSyncTriggers } from './sync/syncTriggers'
+import OfflineBanner from './components/Layout/OfflineBanner'
 
 interface ProtectedRouteProps {
   children: ReactNode
@@ -146,6 +148,11 @@ export default function App() {
     }
   }, [isAuthenticated])
 
+  useEffect(() => {
+    registerSyncTriggers()
+    return () => unregisterSyncTriggers()
+  }, [])
+
   const location = useLocation()
   const isSharedPage = location.pathname.startsWith('/shared/')
 
@@ -178,6 +185,7 @@ export default function App() {
   return (
     <TranslationProvider>
       <ToastContainer />
+      <OfflineBanner />
       <Routes>
         <Route path="/" element={<RootRedirect />} />
         <Route path="/login" element={<LoginPage />} />