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/router.tsx

@@ -3,11 +3,12 @@ import { Navigate, Route, Routes } from "react-router-dom";
 import { AppShell } from "./components/app-shell";
 import { ProtectedRoute } from "./components/protected-route";
 import { DashboardPage } from "./pages/dashboard";
-import { LoginRedirectPage, NotFoundPage, PlaceholderPage } from "./pages/placeholder";
+import { LoginRedirectPage, NotFoundPage } from "./pages/placeholder";
 import { ProfilePage } from "./pages/profile";
 import { ProjectsPage } from "./pages/projects";
 import { GitRepositoriesPage } from "./pages/git-repositories";
 import { SSHKeysPage } from "./pages/ssh-keys";
+import { SettingsPage } from "./pages/settings";
 
 export const AppRouter = () => {
   return (
@@ -26,7 +27,7 @@ export const AppRouter = () => {
         <Route path="projects/:projectId/repositories" element={<GitRepositoriesPage />} />
         <Route path="ssh-keys" element={<SSHKeysPage />} />
         <Route path="profile" element={<ProfilePage />} />
-        <Route path="settings" element={<PlaceholderPage title="Settings" />} />
+        <Route path="settings" element={<SettingsPage />} />
       </Route>
       <Route path="/404" element={<NotFoundPage />} />
       <Route path="*" element={<Navigate to="/404" replace />} />