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/tests/test_pomodoro.py

@@ -91,3 +91,12 @@ def test_reap_closes_intervals_left_running(session, settings):
     assert pomodoro.reap_stale(session, settings) == 1
     session.refresh(row)
     assert row.status == "abandoned"
+
+
+def test_timer_disabled_prevents_start(session, settings):
+    settings.pomodoro.timer_enabled = False
+    settings_service.save_settings(session, settings)
+    with pytest.raises(pomodoro.TimerError, match="view-only"):
+        pomodoro.start(session, settings, kind="work")
+    st = pomodoro.state(session, settings)
+    assert st["timer_enabled"] is False