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/sync/syncTriggers.ts

@@ -0,0 +1,68 @@
+/**
+ * Sync triggers — register event listeners that flush the mutation queue
+ * and/or run a full trip sync based on the connectivity trigger source.
+ *
+ * Trigger matrix:
+ *   window 'online'          → flush mutations + full syncAll (network truly back)
+ *   visibilitychange visible → flush mutations only (avoid hammering server on tab switch)
+ *   periodic 30s             → flush mutations only
+ *   WS reconnect             → flush mutations only (no syncAll — avoids rate-limiter
+ *                              on server restart / socket timeout while already online)
+ *
+ * Call `registerSyncTriggers()` once on app mount.
+ * Call `unregisterSyncTriggers()` on unmount / logout.
+ */
+import { mutationQueue } from './mutationQueue'
+import { tripSyncManager } from './tripSyncManager'
+import { setPreReconnectHook } from '../api/websocket'
+
+const PERIODIC_MS = 30_000
+
+let _intervalId: ReturnType<typeof setInterval> | null = null
+let _registered = false
+
+/** Network came back — flush mutations AND re-seed Dexie for all cacheable trips. */
+function onOnline() {
+  mutationQueue.flush().catch(console.error)
+  tripSyncManager.syncAll().catch(console.error)
+}
+
+/** Tab became visible — flush only; don't trigger a potentially expensive syncAll. */
+function onVisibility() {
+  if (!document.hidden && navigator.onLine) {
+    mutationQueue.flush().catch(console.error)
+  }
+}
+
+/** Periodic heartbeat — drain any lingering pending mutations. */
+function onPeriodic() {
+  if (navigator.onLine) {
+    mutationQueue.flush().catch(console.error)
+  }
+}
+
+export function registerSyncTriggers(): void {
+  if (_registered) return
+  _registered = true
+
+  // WS reconnect: flush mutations only — no syncAll to avoid triggering rate
+  // limiters when the socket drops and reconnects while the device is online.
+  setPreReconnectHook(() => mutationQueue.flush())
+
+  window.addEventListener('online', onOnline)
+  document.addEventListener('visibilitychange', onVisibility)
+  _intervalId = setInterval(onPeriodic, PERIODIC_MS)
+}
+
+export function unregisterSyncTriggers(): void {
+  if (!_registered) return
+  _registered = false
+
+  setPreReconnectHook(null)
+  window.removeEventListener('online', onOnline)
+  document.removeEventListener('visibilitychange', onVisibility)
+  if (_intervalId !== null) {
+    clearInterval(_intervalId)
+    _intervalId = null
+  }
+}