fix(security): stop cross-user offline data leak on shared devices (#1176)

Closes BLOCKER B4 — three reinforcing paths could serve one account's
cached data to the next user on a shared device:

- The Workbox 'api-data' cache keyed trip/user-scoped GETs by URL only
  (cookie-blind). Changed to NetworkOnly; offline reads come from the
  per-user IndexedDB cache via the repo layer instead.
- IndexedDB had no per-user scoping. The Dexie connection is now scoped
  per user (trek-offline-u<id>) behind a Proxy so the ~19 importers keep a
  stable binding; login opens the user DB, logout deletes it and returns
  to the anonymous DB.
- logout() was fire-and-forget and racy: background flush/syncAll could
  re-seed the DB after the wipe. It is now async and ordered — close an
  auth gate, unregister sync triggers, disconnect, clear caches, delete
  the user DB — and flush()/syncAll() bail when the gate is closed.

client/src/sync/authGate.ts

@@ -0,0 +1,18 @@
+/**
+ * Auth gate — a single boolean the sync layer checks before touching the
+ * offline DB. It lets logout disable all background sync (flush / syncAll /
+ * periodic triggers) *before* awaiting the DB swap, so an in-flight loop can't
+ * re-seed the database after the user has logged out.
+ *
+ * Kept separate from authStore to avoid an import cycle
+ * (authStore → tripSyncManager → authStore).
+ */
+let _authed = false
+
+export function setAuthed(value: boolean): void {
+  _authed = value
+}
+
+export function isAuthed(): boolean {
+  return _authed
+}

client/src/sync/mutationQueue.ts

@@ -7,6 +7,7 @@
  */
 import { offlineDb } from '../db/offlineDb'
 import { apiClient } from '../api/client'
+import { isAuthed } from './authGate'
 import type { QueuedMutation } from '../db/offlineDb'
 import type { Table } from 'dexie'
 
@@ -88,7 +89,7 @@ export const mutationQueue = {
    * 4xx responses are marked failed and skipped.
    */
   async flush(): Promise<void> {
-    if (_flushing || !navigator.onLine) return
+    if (_flushing || !navigator.onLine || !isAuthed()) return
     _flushing = true
     // tempId → realId learned during this flush, so a dependent edit/delete
     // queued against an offline-created entity (still holding the negative id)

client/src/sync/tripSyncManager.ts

@@ -29,6 +29,7 @@ import {
   clearTripData,
 } from '../db/offlineDb'
 import { prefetchTilesForTrip } from './tilePrefetcher'
+import { isAuthed } from './authGate'
 import { useSettingsStore } from '../store/settingsStore'
 import type { Trip, Day, Place, PackingItem, TodoItem, BudgetItem, Reservation, TripFile, Accommodation, TripMember } from '../types'
 
@@ -134,7 +135,7 @@ export const tripSyncManager = {
    * No-ops when offline.
    */
   async syncAll(): Promise<void> {
-    if (_syncing || !navigator.onLine) return
+    if (_syncing || !navigator.onLine || !isAuthed()) return
     _syncing = true
     try {
       const { trips } = await tripsApi.list() as { trips: Trip[] }