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.

tests/unit/test_ui_routes.py

@@ -4,6 +4,7 @@ import pytest
 from fastapi.testclient import TestClient
 
 from models.scenario import Scenario
+from services import settings as settings_service
 
 
 def test_dashboard_route_provides_summary(
@@ -129,3 +130,36 @@ def test_additional_ui_routes_render_templates(
 
     context = cast(Dict[str, Any], getattr(response, "context", {}))
     assert context
+
+
+def test_settings_route_provides_css_context(
+    api_client: TestClient,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    env_var = settings_service.css_key_to_env_var("--color-accent")
+    monkeypatch.setenv(env_var, "#abcdef")
+
+    response = api_client.get("/ui/settings")
+    assert response.status_code == 200
+
+    template = getattr(response, "template", None)
+    assert template is not None
+    assert template.name == "settings.html"
+
+    context = cast(Dict[str, Any], getattr(response, "context", {}))
+    assert "css_variables" in context
+    assert "css_defaults" in context
+    assert "css_env_overrides" in context
+    assert "css_env_override_rows" in context
+    assert "css_env_override_meta" in context
+
+    assert context["css_variables"]["--color-accent"] == "#abcdef"
+    assert context["css_defaults"]["--color-accent"] == settings_service.CSS_COLOR_DEFAULTS["--color-accent"]
+    assert context["css_env_overrides"]["--color-accent"] == "#abcdef"
+
+    override_rows = context["css_env_override_rows"]
+    assert any(row["env_var"] == env_var for row in override_rows)
+
+    meta = context["css_env_override_meta"]["--color-accent"]
+    assert meta["value"] == "#abcdef"
+    assert meta["env_var"] == env_var