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/db/migrations.ts

@@ -1578,6 +1578,22 @@ function runMigrations(db: Database.Database): void {
     () => {
       try { db.exec('ALTER TABLE journey_contributors ADD COLUMN hide_skeletons INTEGER NOT NULL DEFAULT 0'); } catch {}
     },
+    // Migration 100: Idempotency keys for offline mutation replay
+    () => {
+      db.exec(`
+        CREATE TABLE IF NOT EXISTS idempotency_keys (
+          key         TEXT NOT NULL,
+          user_id     INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+          method      TEXT NOT NULL,
+          path        TEXT NOT NULL,
+          status_code INTEGER NOT NULL,
+          response_body TEXT NOT NULL,
+          created_at  INTEGER NOT NULL DEFAULT (strftime('%s','now')),
+          PRIMARY KEY (key, user_id)
+        );
+        CREATE INDEX IF NOT EXISTS idx_idempotency_keys_created ON idempotency_keys(created_at);
+      `);
+    },
   ];
 
   if (currentVersion < migrations.length) {