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/pages/JourneyDetailPage.test.tsx

@@ -17,6 +17,7 @@ vi.mock('../api/websocket', () => ({
   disconnect: vi.fn(),
   getSocketId: vi.fn(() => null),
   setRefetchCallback: vi.fn(),
+  setPreReconnectHook: vi.fn(),
   addListener: vi.fn(),
   removeListener: vi.fn(),
 }));

client/src/pages/SettingsPage.tsx

@@ -11,6 +11,7 @@ import NotificationsTab from '../components/Settings/NotificationsTab'
 import IntegrationsTab from '../components/Settings/IntegrationsTab'
 import AccountTab from '../components/Settings/AccountTab'
 import AboutTab from '../components/Settings/AboutTab'
+import OfflineTab from '../components/Settings/OfflineTab'
 
 export default function SettingsPage(): React.ReactElement {
   const { t } = useTranslation()
@@ -41,6 +42,7 @@ export default function SettingsPage(): React.ReactElement {
     { id: 'map', label: t('settings.tabs.map') },
     { id: 'notifications', label: t('settings.tabs.notifications') },
     ...(hasIntegrations ? [{ id: 'integrations', label: t('settings.tabs.integrations') }] : []),
+    { id: 'offline', label: t('settings.tabs.offline', 'Offline') },
     { id: 'account', label: t('settings.tabs.account') },
     ...(appVersion ? [{ id: 'about', label: t('settings.tabs.about') }] : []),
   ]
@@ -84,6 +86,7 @@ export default function SettingsPage(): React.ReactElement {
           {activeTab === 'map' && <MapSettingsTab />}
           {activeTab === 'notifications' && <NotificationsTab />}
           {activeTab === 'integrations' && hasIntegrations && <IntegrationsTab />}
+          {activeTab === 'offline' && <OfflineTab />}
           {activeTab === 'account' && <AccountTab />}
           {activeTab === 'about' && appVersion && <AboutTab appVersion={appVersion} />}
         </div>