Refactor the signal handling for the camera start/stop button in `MainController` to simplify logic and improve reliability.
- Replaced the dynamic connect/disconnect pattern with a single, persistent signal connection to `_on_start_button_clicked`.
- Centralized UI state updates (e.g., button text) into a new `_update_start_button_state` method.
- This eliminates potential errors from mismatched signal connections and makes the control flow easier to follow.
- Also fixes major indentation errors caused by a previous faulty replacement.
