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/web/src/components/app-shell.tsx

@@ -1,5 +1,6 @@
 import { Link, NavLink, Outlet } from "react-router-dom";
 
+import { useTheme } from "../hooks/use-theme";
 import { useAuth } from "../state/auth";
 
 const NAV_ITEMS = [
@@ -10,6 +11,7 @@ const NAV_ITEMS = [
 ];
 
 export const AppShell = () => {
+  useTheme();
   const { user, logout } = useAuth();
 
   return (