feat: complete user config management

- Add theme support with dark/light/system modes
- Add useTheme hook for applying user config theme
- Update router to use SettingsPage
- Update app-shell to apply theme on load
- Add CSS variables for dark theme
- Fix mypy errors in user_config.py
- Quality gates pass: ruff, mypy, typecheck, lint, build

apps/api/src/main.py

@@ -5,6 +5,7 @@ from src.api.auth import router as auth_router
 from src.api.git_repositories import router as git_repositories_router
 from src.api.projects import router as projects_router
 from src.api.ssh_keys import router as ssh_keys_router
+from src.api.user_config import router as user_config_router
 from src.api.users import router as users_router
 
 app = FastAPI(title="Headquarter API")
@@ -13,4 +14,5 @@ app.include_router(projects_router)
 app.include_router(users_router)
 app.include_router(ssh_keys_router)
 app.include_router(git_repositories_router)
+app.include_router(user_config_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")