refactor: enhance TimerPage layout and functionality; update theme and tokens for improved design consistency

- Refactored TimerPage component to improve layout and user experience.
- Updated theme configuration to utilize design tokens for better maintainability.
- Introduced new tokens for colors and typography to align with design principles.
- Simplified state management and improved timer functionality.
- Enhanced accessibility and responsiveness of UI elements.

backend/app/sources/base.py

@@ -63,8 +63,5 @@ class TodoSource(Protocol):
     def fetch(self) -> list[TodoItem]:
         """Every open todo this source knows about."""
 
-    def complete(self, item: TodoItem) -> bool:
-        """Close the task in the external system. False = not supported."""
-
     def check(self) -> str:
         """Verify credentials; return a short human description of the account."""