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/repo/packingRepo.test.ts

@@ -66,38 +66,28 @@ describe('packingRepo.list', () => {
 });
 
 describe('packingRepo.create', () => {
-  it('calls REST and caches created item in Dexie', async () => {
-    const item = buildPackingItem({ trip_id: 1, name: 'Sunscreen' });
-    server.use(
-      http.post('/api/trips/1/packing', () => HttpResponse.json({ item })),
-    );
-
+  it('writes item optimistically to Dexie immediately', async () => {
     const result = await packingRepo.create(1, { name: 'Sunscreen' });
     expect(result.item.name).toBe('Sunscreen');
+    // tempId is negative (-(Date.now()))
+    expect(result.item.id).toBeLessThan(0);
 
-    await new Promise(r => setTimeout(r, 0));
-    const cached = await offlineDb.packingItems.get(item.id);
-    expect(cached).toBeDefined();
-    expect(cached!.name).toBe('Sunscreen');
+    const cached = await offlineDb.packingItems.where('trip_id').equals(1).toArray();
+    expect(cached).toHaveLength(1);
+    expect(cached[0].name).toBe('Sunscreen');
   });
 });
 
 describe('packingRepo.update', () => {
-  it('calls REST and updates Dexie cache', async () => {
+  it('writes optimistic update to Dexie immediately', async () => {
     const original = buildPackingItem({ trip_id: 1, name: 'Jacket', checked: 0 });
     await offlineDb.packingItems.put(original);
 
-    const updated = { ...original, checked: 1 };
-    server.use(
-      http.put(`/api/trips/1/packing/${original.id}`, () => HttpResponse.json({ item: updated })),
-    );
-
     const result = await packingRepo.update(1, original.id, { checked: true });
-    expect(result.item.checked).toBe(1);
+    expect(result.item.checked).toBeTruthy();
 
-    await new Promise(r => setTimeout(r, 0));
     const cached = await offlineDb.packingItems.get(original.id);
-    expect(cached!.checked).toBe(1);
+    expect(cached!.checked).toBeTruthy();
   });
 });