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/tests/unit/sync/tripSyncManager.test.ts

@@ -9,6 +9,7 @@ import 'fake-indexeddb/auto';
 import { server } from '../../helpers/msw/server';
 import { http, HttpResponse } from 'msw';
 import { tripSyncManager } from '../../../src/sync/tripSyncManager';
+import { setAuthed } from '../../../src/sync/authGate';
 import { offlineDb, clearAll, upsertTrip } from '../../../src/db/offlineDb';
 import {
   buildTrip,
@@ -45,6 +46,7 @@ function makeBundle(tripId: number) {
 beforeEach(async () => {
   await clearAll();
   tripSyncManager._resetSyncing();
+  setAuthed(true);
   Object.defineProperty(navigator, 'onLine', { value: true, writable: true, configurable: true });
   // Stub fetch for blob caching (used by cacheFilesForTrip)
   vi.stubGlobal('fetch', vi.fn().mockResolvedValue({
@@ -56,6 +58,19 @@ beforeEach(async () => {
 afterEach(() => {
   vi.restoreAllMocks();
   vi.unstubAllGlobals();
+  setAuthed(false);
+});
+
+describe('tripSyncManager.syncAll — auth gate (B4)', () => {
+  it('no-ops when logged out (gate closed)', async () => {
+    setAuthed(false);
+    let called = false;
+    server.use(
+      http.get('/api/trips', () => { called = true; return HttpResponse.json({ trips: [] }); }),
+    );
+    await tripSyncManager.syncAll();
+    expect(called).toBe(false);
+  });
 });
 
 // ── offline guard ─────────────────────────────────────────────────────────────