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/store/authStore.ts

@@ -4,6 +4,8 @@ import { authApi } from '../api/client'
 import { connect, disconnect } from '../api/websocket'
 import type { User } from '../types'
 import { getApiErrorMessage } from '../types'
+import { tripSyncManager } from '../sync/tripSyncManager'
+import { clearAll } from '../db/offlineDb'
 
 interface AuthResponse {
   user: User
@@ -88,6 +90,7 @@ export const useAuthStore = create<AuthState>()(
         error: null,
       })
       connect()
+      tripSyncManager.syncAll().catch(console.error)
       return data as AuthResponse
     } catch (err: unknown) {
       const error = getApiErrorMessage(err, 'Login failed')
@@ -108,6 +111,7 @@ export const useAuthStore = create<AuthState>()(
         error: null,
       })
       connect()
+      tripSyncManager.syncAll().catch(console.error)
       return data as AuthResponse
     } catch (err: unknown) {
       const error = getApiErrorMessage(err, 'Verification failed')
@@ -128,6 +132,7 @@ export const useAuthStore = create<AuthState>()(
         error: null,
       })
       connect()
+      tripSyncManager.syncAll().catch(console.error)
       return data
     } catch (err: unknown) {
       const error = getApiErrorMessage(err, 'Registration failed')
@@ -145,6 +150,8 @@ export const useAuthStore = create<AuthState>()(
       caches.delete('api-data').catch(() => {})
       caches.delete('user-uploads').catch(() => {})
     }
+    // Purge all cached trip data from IndexedDB
+    clearAll().catch(console.error)
     set({
       user: null,
       isAuthenticated: false,