Implement the logic for capturing and saving photos from the live preview stream.
- Add `save_photo` method to `MediaRepository` to handle file saving and database updates.
- `MainController` now tracks the selected color and the latest camera frame.
- The "Take Photo" button is enabled only when a color is selected.
- Pressing the button saves the current preview frame to the correct media folder and refreshes the thumbnail list.
- Fixes indentation issues in `main_controller.py` caused by previous faulty replacements.
Refactor the database handling to use a Singleton pattern for `DatabaseManager`.
- A single, module-level instance `db_manager` is created in `core/database.py` to ensure one database connection is used throughout the application.
- `MediaRepository` and `MainController` are updated to use this shared instance instead of creating their own.
- This simplifies dependency injection and prevents potential issues with multiple database connections.
- Also, update `review.md` to reflect the progress.
