feat: Add application-level settings for CSS color management

- Introduced a new table `application_setting` to store configurable application options.
- Implemented functions to manage CSS color settings, including loading, updating, and reading environment overrides.
- Added a new settings view to render and manage theme colors.
- Updated UI to include a settings page with theme color management and environment overrides display.
- Enhanced CSS styles for the settings page and sidebar navigation.
- Created unit and end-to-end tests for the new settings functionality and CSS management.

models/__init__.py

@@ -1,5 +1,6 @@
 """
-models package initializer. Import the currency model so it's registered
+models package initializer. Import key models so they're registered
 with the shared Base.metadata when the package is imported by tests.
 """
+from . import application_setting  # noqa: F401
 from . import currency  # noqa: F401