fix: remove navigator.onLine guards and fix upsert races in all repos

navigator.onLine returns false transiently during service worker activation
(skipWaiting + clientsClaim), causing all repo refresh IIFEs to return null
immediately on first page load — leaving the UI with empty data until F5.

Fixes applied across all list repos (trip, day, place, packing, todo, budget,
reservation, accommodation, file):
- Drop navigator.onLine guard; let fetch fail naturally when truly offline
- Await all upsert calls (some were fire-and-forget, risking race conditions
  against subsequent reads and silent swallowed failures)
- Return Promise.resolve(null) instead of Promise.resolve(fresh) in the
  IDB-empty network path, so loadTrip's background refresh Promise.all
  resolves null and skips set({trip}), preventing a spurious reference change
  that was resetting the 1500ms splash timer

Tests updated: placeRepo and packingRepo "empty cache" tests now simulate
genuine network failure (HttpResponse.error) instead of relying on the
navigator.onLine guard that no longer exists; DashboardPage tests clear IDB
before each test and use a query-safe assertion after background refresh.

client/src/pages/DashboardPage.test.tsx

@@ -7,10 +7,12 @@ import { resetAllStores, seedStore } from '../../tests/helpers/store';
 import { buildUser, buildAdmin, buildTrip } from '../../tests/helpers/factories';
 import { useAuthStore } from '../store/authStore';
 import { usePermissionsStore } from '../store/permissionsStore';
+import { offlineDb } from '../db/offlineDb';
 import DashboardPage from './DashboardPage';
 
-beforeEach(() => {
+beforeEach(async () => {
   vi.clearAllMocks();
+  await Promise.all(offlineDb.tables.map(t => t.clear()));
   resetAllStores();
   // Seed auth with authenticated user
   seedStore(useAuthStore, { isAuthenticated: true, user: buildUser() });
@@ -329,7 +331,8 @@ describe('DashboardPage', () => {
       const tokyoTrip = screen.getAllByText('Tokyo Trip')[0];
       await user.click(tokyoTrip);
 
-      expect(tokyoTrip).toBeInTheDocument();
+      // Re-query after click — background refresh may re-render the list
+      expect(screen.getAllByText('Tokyo Trip').length).toBeGreaterThan(0);
     });
   });