feat: always-optimistic write pattern across all repos

All create/update/delete repo methods now write to IndexedDB optimistically
and fire mutationQueue.flush() as fire-and-forget, returning immediately
without waiting for the network. This eliminates the 8-second UX freeze
previously seen when the API was unreachable but navigator.onLine was true.

- Repos rewritten: trip, day, place, packing, todo, budget, accommodation,
  reservation, file — write methods never throw, always return optimistic data
- mutationQueue.flush() changed to iterative (one item per loop iteration)
  so mutations enqueued mid-flush (e.g. bulk check-all) are picked up
- fileRepo.toggleStar skips the IDB put when the file is not cached locally
- DayDetailPanel passes place_name into accommodationRepo.create so the
  optimistic accommodation renders the correct hotel label immediately
- Test suite updated throughout to reflect optimistic-first semantics:
  no more rollback assertions, IDB cleared in component test beforeEach hooks,
  FileManager tests switched from filesApi spy to MSW endpoint assertions

client/src/components/Packing/PackingListPanel.test.tsx

@@ -9,8 +9,11 @@ import { useTripStore } from '../../store/tripStore';
 import { resetAllStores, seedStore } from '../../../tests/helpers/store';
 import { buildUser, buildTrip, buildPackingItem } from '../../../tests/helpers/factories';
 import PackingListPanel from './PackingListPanel';
+import { offlineDb } from '../../db/offlineDb';
 
-beforeEach(() => {
+beforeEach(async () => {
+  await new Promise<void>(resolve => setTimeout(resolve, 0));
+  await Promise.all(offlineDb.tables.map(t => t.clear()));
   resetAllStores();
   // Side-effect APIs PackingListPanel calls on mount
   server.use(