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/api/client.ts

@@ -38,13 +38,25 @@ export const apiClient: AxiosInstance = axios.create({
   },
 })
 
-// Request interceptor - add socket ID
+const MUTATING_METHODS = new Set(['post', 'put', 'patch', 'delete'])
+
+// Request interceptor - add socket ID + idempotency key for mutating requests
 apiClient.interceptors.request.use(
   (config) => {
     const sid = getSocketId()
     if (sid) {
       config.headers['X-Socket-Id'] = sid
     }
+    // Attach a per-request idempotency key to all write operations so the
+    // server can deduplicate retried requests (e.g. network blips).
+    // The mutation queue sets its own pre-generated key; skip if already set.
+    const method = (config.method ?? '').toLowerCase()
+    if (MUTATING_METHODS.has(method) && !config.headers['X-Idempotency-Key']) {
+      const key = typeof crypto !== 'undefined' && crypto.randomUUID
+        ? crypto.randomUUID()
+        : Math.random().toString(36).slice(2)
+      config.headers['X-Idempotency-Key'] = key
+    }
     return config
   },
   (error) => Promise.reject(error)
@@ -161,6 +173,7 @@ export const tripsApi = {
   addMember: (id: number | string, identifier: string) => apiClient.post(`/trips/${id}/members`, { identifier }).then(r => r.data),
   removeMember: (id: number | string, userId: number) => apiClient.delete(`/trips/${id}/members/${userId}`).then(r => r.data),
   copy: (id: number | string, data?: { title?: string }) => apiClient.post(`/trips/${id}/copy`, data || {}).then(r => r.data),
+  bundle: (id: number | string) => apiClient.get(`/trips/${id}/bundle`).then(r => r.data),
 }
 
 export const daysApi = {