Refactor: replace `EventManager` with `DefaultEventRepositoryMediator` for improved coordination logic

- Removed `EventManager` and introduced `DefaultEventRepositoryMediator` to streamline network and local repository communication.
- Updated `DataModule` to provide `DefaultEventRepositoryMediator` and a new `EventManagerRepositoryImpl`.
- Enhanced error handling for repository operations and unified event creation, update, and deletion logic.
- Added validation to `EventInfo` to ensure `startTime` is before `finishTime`.
- Simplified `UpdateUseCase` with a consistent `DEFAULT_DELAY` for better readability and maintainability.
- Refined time rounding logic in `NetworkEventRepository` for accurate event scheduling updates.
- Updated `iosApp.xcscheme` to include new configurations.