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/tests/unit/tripStore.test.ts

@@ -219,8 +219,8 @@ describe('tripStore', () => {
 
       const result = await useTripStore.getState().updateTrip(1, { name: 'Updated Trip' });
 
-      expect(result).toEqual(updatedTrip);
-      expect(useTripStore.getState().trip).toEqual(updatedTrip);
+      expect(result.name).toBe('Updated Trip');
+      expect(useTripStore.getState().trip?.name).toBe('Updated Trip');
     });
   });