feat: 添加日历批量操作与客户端时区感知功能，优化前端 UI 交互体验

backend/tests/unit/v1/schedule_items/test_service.py

@@ -148,6 +148,7 @@ async def test_create_success(
     request = ScheduleItemCreateRequest(
         title="Test Event",
         start_at=datetime(2026, 2, 28, 16, 0, 0, tzinfo=timezone.utc),
+        timezone="UTC",
     )
     service = ScheduleItemService(
         repository=FakeRepo(None),
@@ -171,6 +172,7 @@ async def test_create_invalid_end_at(
         title="Test Event",
         start_at=datetime(2026, 2, 28, 17, 0, 0, tzinfo=timezone.utc),
         end_at=datetime(2026, 2, 28, 16, 0, 0, tzinfo=timezone.utc),
+        timezone="UTC",
     )
     service = ScheduleItemService(
         repository=FakeRepo(None),
@@ -275,6 +277,7 @@ async def test_create_maps_metadata_to_extra_metadata(
     request = ScheduleItemCreateRequest(
         title="Roadmap",
         start_at=datetime(2026, 2, 28, 16, 0, 0, tzinfo=timezone.utc),
+        timezone="UTC",
         metadata=ScheduleItemMetadata(
             location="会议室A",
             color="#4F46E5",