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/components/Layout/BottomNav.test.tsx

@@ -5,6 +5,7 @@ vi.mock('../../api/websocket', () => ({
   disconnect: vi.fn(),
   getSocketId: vi.fn(() => null),
   setRefetchCallback: vi.fn(),
+  setPreReconnectHook: vi.fn(),
   addListener: vi.fn(),
   removeListener: vi.fn(),
 }));

client/src/components/Layout/OfflineBanner.tsx

@@ -0,0 +1,83 @@
+/**
+ * OfflineBanner — persistent top bar indicating connectivity + sync state.
+ *
+ * States:
+ *   offline + N queued  →  amber bar "Offline — N changes queued"
+ *   offline + 0 queued  →  amber bar "Offline"
+ *   online  + N pending →  blue bar  "Syncing N changes…"
+ *   online  + 0 pending →  hidden
+ */
+import React, { useState, useEffect } from 'react'
+import { WifiOff, RefreshCw } from 'lucide-react'
+import { mutationQueue } from '../../sync/mutationQueue'
+
+const POLL_MS = 3_000
+
+export default function OfflineBanner(): React.ReactElement | null {
+  const [isOnline, setIsOnline] = useState(navigator.onLine)
+  const [pendingCount, setPendingCount] = useState(0)
+
+  useEffect(() => {
+    const onOnline  = () => setIsOnline(true)
+    const onOffline = () => setIsOnline(false)
+    window.addEventListener('online',  onOnline)
+    window.addEventListener('offline', onOffline)
+    return () => {
+      window.removeEventListener('online',  onOnline)
+      window.removeEventListener('offline', onOffline)
+    }
+  }, [])
+
+  useEffect(() => {
+    let cancelled = false
+    async function poll() {
+      const n = await mutationQueue.pendingCount()
+      if (!cancelled) setPendingCount(n)
+    }
+    poll()
+    const id = setInterval(poll, POLL_MS)
+    return () => { cancelled = true; clearInterval(id) }
+  }, [])
+
+  const hidden = isOnline && pendingCount === 0
+  if (hidden) return null
+
+  const offline = !isOnline
+  const bg    = offline ? '#92400e' : '#1e40af'
+  const text  = '#fff'
+
+  const label = offline
+    ? pendingCount > 0
+      ? `Offline — ${pendingCount} change${pendingCount !== 1 ? 's' : ''} queued`
+      : 'Offline'
+    : `Syncing ${pendingCount} change${pendingCount !== 1 ? 's' : ''}…`
+
+  return (
+    <div
+      role="status"
+      aria-live="polite"
+      style={{
+        position: 'fixed',
+        top: 0,
+        left: 0,
+        right: 0,
+        zIndex: 9999,
+        background: bg,
+        color: text,
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+        gap: 8,
+        padding: '6px 16px',
+        fontSize: 13,
+        fontWeight: 500,
+      }}
+    >
+      {offline
+        ? <WifiOff size={14} />
+        : <RefreshCw size={14} style={{ animation: 'spin 1s linear infinite' }} />
+      }
+      {label}
+    </div>
+  )
+}