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/todoist.py

@@ -117,18 +117,6 @@ class TodoistSource:
         projects = self._projects()
         return f"{len(projects)} Todoist project(s) visible"
 
-    def complete(self, item: TodoItem) -> bool:
-        """`POST /tasks/{id}/close` — the same verb the apps' checkbox uses."""
-        if not self._config.writeback_done:
-            return False
-        try:
-            resp = self._client.post(f"/tasks/{item.external_id}/close")
-        except httpx.HTTPError as exc:
-            raise SourceError(f"Todoist is unreachable: {exc}") from exc
-        if not resp.is_success:
-            raise SourceError(f"Todoist could not close the task: {resp.status_code}")
-        return True
-
     def close(self) -> None:
         self._client.close()