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/db/offlineDb.ts

@@ -0,0 +1,156 @@
+import Dexie, { type Table } from 'dexie';
+import type { Trip, Day, Place, PackingItem, TodoItem, BudgetItem, Reservation, TripFile } from '../types';
+
+// ── Queue + sync types ────────────────────────────────────────────────────────
+
+export type MutationStatus = 'pending' | 'syncing' | 'failed';
+
+export interface QueuedMutation {
+  /** UUID — also used as X-Idempotency-Key sent to the server */
+  id: string;
+  tripId: number;
+  method: 'POST' | 'PUT' | 'PATCH' | 'DELETE';
+  url: string;
+  body: unknown;
+  createdAt: number;
+  status: MutationStatus;
+  attempts: number;
+  lastError: string | null;
+  /** Dexie table name to write the server response into after flush (e.g. 'places') */
+  resource?: string;
+  /** For CREATE mutations enqueued offline: the temporary negative id written to Dexie */
+  tempId?: number;
+  /** For DELETE mutations: the entity id to remove from Dexie on flush */
+  entityId?: number;
+}
+
+export interface SyncMeta {
+  tripId: number;
+  lastSyncedAt: number | null;
+  status: 'idle' | 'syncing' | 'error';
+  /** Bounding box [minLng, minLat, maxLng, maxLat] of pre-downloaded map tiles */
+  tilesBbox: [number, number, number, number] | null;
+  filesCachedCount: number;
+}
+
+export interface BlobCacheEntry {
+  /** Relative URL, e.g. "/api/files/42/download" */
+  url: string;
+  blob: Blob;
+  mime: string;
+  cachedAt: number;
+}
+
+// ── Dexie class ────────────────────────────────────────────────────────────────
+
+class TrekOfflineDb extends Dexie {
+  trips!: Table<Trip, number>;
+  days!: Table<Day, number>;
+  places!: Table<Place, number>;
+  packingItems!: Table<PackingItem, number>;
+  todoItems!: Table<TodoItem, number>;
+  budgetItems!: Table<BudgetItem, number>;
+  reservations!: Table<Reservation, number>;
+  tripFiles!: Table<TripFile, number>;
+  mutationQueue!: Table<QueuedMutation, string>;
+  syncMeta!: Table<SyncMeta, number>;
+  blobCache!: Table<BlobCacheEntry, string>;
+
+  constructor() {
+    super('trek-offline');
+
+    this.version(1).stores({
+      trips:        'id',
+      days:         'id, trip_id',
+      places:       'id, trip_id',
+      packingItems: 'id, trip_id',
+      todoItems:    'id, trip_id',
+      budgetItems:  'id, trip_id',
+      reservations: 'id, trip_id',
+      tripFiles:    'id, trip_id',
+      mutationQueue:'id, tripId, status, createdAt',
+      syncMeta:     'tripId',
+      blobCache:    'url, cachedAt',
+    });
+  }
+}
+
+export const offlineDb = new TrekOfflineDb();
+
+// ── Bulk upsert helpers ────────────────────────────────────────────────────────
+
+export async function upsertTrip(trip: Trip): Promise<void> {
+  await offlineDb.trips.put(trip);
+}
+
+export async function upsertDays(days: Day[]): Promise<void> {
+  await offlineDb.days.bulkPut(days);
+}
+
+export async function upsertPlaces(places: Place[]): Promise<void> {
+  await offlineDb.places.bulkPut(places);
+}
+
+export async function upsertPackingItems(items: PackingItem[]): Promise<void> {
+  await offlineDb.packingItems.bulkPut(items);
+}
+
+export async function upsertTodoItems(items: TodoItem[]): Promise<void> {
+  await offlineDb.todoItems.bulkPut(items);
+}
+
+export async function upsertBudgetItems(items: BudgetItem[]): Promise<void> {
+  await offlineDb.budgetItems.bulkPut(items);
+}
+
+export async function upsertReservations(items: Reservation[]): Promise<void> {
+  await offlineDb.reservations.bulkPut(items);
+}
+
+export async function upsertTripFiles(files: TripFile[]): Promise<void> {
+  await offlineDb.tripFiles.bulkPut(files);
+}
+
+export async function upsertSyncMeta(meta: SyncMeta): Promise<void> {
+  await offlineDb.syncMeta.put(meta);
+}
+
+// ── Eviction / cleanup ────────────────────────────────────────────────────────
+
+/** Delete all cached data for one trip (eviction or explicit clear). */
+export async function clearTripData(tripId: number): Promise<void> {
+  await offlineDb.transaction(
+    'rw',
+    [
+      offlineDb.days,
+      offlineDb.places,
+      offlineDb.packingItems,
+      offlineDb.todoItems,
+      offlineDb.budgetItems,
+      offlineDb.reservations,
+      offlineDb.tripFiles,
+      offlineDb.mutationQueue,
+      offlineDb.syncMeta,
+    ],
+    async () => {
+      await offlineDb.days.where('trip_id').equals(tripId).delete();
+      await offlineDb.places.where('trip_id').equals(tripId).delete();
+      await offlineDb.packingItems.where('trip_id').equals(tripId).delete();
+      await offlineDb.todoItems.where('trip_id').equals(tripId).delete();
+      await offlineDb.budgetItems.where('trip_id').equals(tripId).delete();
+      await offlineDb.reservations.where('trip_id').equals(tripId).delete();
+      await offlineDb.tripFiles.where('trip_id').equals(tripId).delete();
+      await offlineDb.mutationQueue.where('tripId').equals(tripId).delete();
+      await offlineDb.syncMeta.where('tripId').equals(tripId).delete();
+    },
+  );
+  // Remove the trip row itself outside the transaction since it's a separate table
+  await offlineDb.trips.delete(tripId);
+}
+
+/** Wipe the entire offline database (called on logout). */
+export async function clearAll(): Promise<void> {
+  await offlineDb.delete();
+  // Re-open so subsequent operations don't fail
+  await offlineDb.open();
+}