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

server/src/scheduler.ts

@@ -230,11 +230,35 @@ function startVersionCheck(): void {
   }, { timezone: tz });
 }
 
+// Idempotency key cleanup: nightly at 3 AM — delete keys older than 24 hours
+let idempotencyCleanupTask: ScheduledTask | null = null;
+
+function startIdempotencyCleanup(): void {
+  if (idempotencyCleanupTask) { idempotencyCleanupTask.stop(); idempotencyCleanupTask = null; }
+
+  const tz = process.env.TZ || 'UTC';
+  idempotencyCleanupTask = cron.schedule('0 3 * * *', () => {
+    try {
+      const { db } = require('./db/database');
+      const cutoff = Math.floor(Date.now() / 1000) - 86400;
+      const result = db.prepare('DELETE FROM idempotency_keys WHERE created_at < ?').run(cutoff);
+      if (result.changes > 0) {
+        const { logInfo: li } = require('./services/auditLog');
+        li(`Idempotency cleanup: removed ${result.changes} expired key(s)`);
+      }
+    } catch (err: unknown) {
+      const { logError: le } = require('./services/auditLog');
+      le(`Idempotency cleanup: ${err instanceof Error ? err.message : err}`);
+    }
+  }, { timezone: tz });
+}
+
 function stop(): void {
   if (currentTask) { currentTask.stop(); currentTask = null; }
   if (demoTask) { demoTask.stop(); demoTask = null; }
   if (reminderTask) { reminderTask.stop(); reminderTask = null; }
   if (versionCheckTask) { versionCheckTask.stop(); versionCheckTask = null; }
+  if (idempotencyCleanupTask) { idempotencyCleanupTask.stop(); idempotencyCleanupTask = null; }
 }
 
-export { start, stop, startDemoReset, startTripReminders, startVersionCheck, loadSettings, saveSettings, VALID_INTERVALS };
+export { start, stop, startDemoReset, startTripReminders, startVersionCheck, startIdempotencyCleanup, loadSettings, saveSettings, VALID_INTERVALS };