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/tests/unit/repo/packingRepo.test.ts

@@ -58,8 +58,10 @@ describe('packingRepo.list', () => {
     expect(restCalled).toBe(false);
   });
 
-  it('offline — returns empty array when nothing cached', async () => {
-    Object.defineProperty(navigator, 'onLine', { value: false });
+  it('offline — returns empty array when nothing cached and network fails', async () => {
+    server.use(
+      http.get('/api/trips/99/packing', () => HttpResponse.error()),
+    );
     const result = await packingRepo.list(99);
     expect(result.items).toHaveLength(0);
   });

client/tests/unit/repo/placeRepo.test.ts

@@ -59,8 +59,10 @@ describe('placeRepo.list', () => {
     expect(restCalled).toBe(false);
   });
 
-  it('offline — returns empty array when nothing cached', async () => {
-    Object.defineProperty(navigator, 'onLine', { value: false });
+  it('offline — returns empty array when nothing cached and network fails', async () => {
+    server.use(
+      http.get('/api/trips/99/places', () => HttpResponse.error()),
+    );
     const result = await placeRepo.list(99);
     expect(result.places).toHaveLength(0);
   });