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/services/pomodoro.py

@@ -115,7 +115,9 @@ def start(
     task_id: int | None = None,
     note: str | None = None,
 ) -> PomodoroSession:
-    """Begin an interval, abandoning whatever was still running."""
+    if not settings.pomodoro.timer_enabled:
+        raise TimerError("Timer is disabled (view-only mode).")
+
     running = current_session(session)
     if running is not None:
         _finish(session, settings, running, status="abandoned", reason="superseded")
@@ -317,6 +319,7 @@ def state(session: Session, settings: AppSettings) -> dict[str, Any]:
             "long_break": settings.pomodoro.long_break_minutes,
         },
         "server_time": utcnow().isoformat(),
+        "timer_enabled": settings.pomodoro.timer_enabled,
     }