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/hooks/usePendingMutations.ts

@@ -0,0 +1,43 @@
+/**
+ * usePendingMutations — returns the set of entity IDs that have a pending
+ * or syncing mutation for a given trip.
+ *
+ * Components use this to render a clock/pending indicator on list rows.
+ * Polls Dexie every 2 s so the indicator clears automatically once synced.
+ */
+import { useState, useEffect } from 'react'
+import { mutationQueue } from '../sync/mutationQueue'
+
+const POLL_MS = 2_000
+
+export function usePendingMutations(tripId: number): Set<number> {
+  const [pendingIds, setPendingIds] = useState<Set<number>>(new Set())
+
+  useEffect(() => {
+    let cancelled = false
+
+    async function refresh() {
+      const pending = await mutationQueue.pending(tripId)
+      if (cancelled) return
+
+      const ids = new Set<number>()
+      for (const m of pending) {
+        // Extract entity id from the mutation URL (last numeric segment)
+        const match = m.url.match(/\/(\d+)$/)
+        if (match) ids.add(Number(match[1]))
+        // Also include tempId for offline-created items
+        if (m.tempId !== undefined) ids.add(m.tempId)
+      }
+      setPendingIds(ids)
+    }
+
+    refresh()
+    const timer = setInterval(refresh, POLL_MS)
+    return () => {
+      cancelled = true
+      clearInterval(timer)
+    }
+  }, [tripId])
+
+  return pendingIds
+}