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_settings_routes.py

@@ -0,0 +1,53 @@
+import pytest
+from fastapi.testclient import TestClient
+from sqlalchemy.orm import Session
+
+from services import settings as settings_service
+
+
+@pytest.mark.usefixtures("db_session")
+def test_read_css_settings_reflects_env_overrides(
+    api_client: TestClient, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    env_var = settings_service.css_key_to_env_var("--color-background")
+    monkeypatch.setenv(env_var, "#123456")
+
+    response = api_client.get("/api/settings/css")
+    assert response.status_code == 200
+    body = response.json()
+
+    assert body["variables"]["--color-background"] == "#123456"
+    assert body["env_overrides"]["--color-background"] == "#123456"
+    assert any(
+        source["env_var"] == env_var and source["value"] == "#123456"
+        for source in body["env_sources"]
+    )
+
+
+@pytest.mark.usefixtures("db_session")
+def test_update_css_settings_persists_changes(
+    api_client: TestClient, db_session: Session
+) -> None:
+    payload = {"variables": {"--color-primary": "#112233"}}
+
+    response = api_client.put("/api/settings/css", json=payload)
+    assert response.status_code == 200
+    body = response.json()
+
+    assert body["variables"]["--color-primary"] == "#112233"
+
+    persisted = settings_service.get_css_color_settings(db_session)
+    assert persisted["--color-primary"] == "#112233"
+
+
+@pytest.mark.usefixtures("db_session")
+def test_update_css_settings_invalid_value_returns_422(
+    api_client: TestClient
+) -> None:
+    response = api_client.put(
+        "/api/settings/css",
+        json={"variables": {"--color-primary": "not-a-color"}},
+    )
+    assert response.status_code == 422
+    body = response.json()
+    assert "color" in body["detail"].lower()