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/api/settings.ts

@@ -0,0 +1,25 @@
+import { apiClient } from "./client";
+
+export interface UserConfig {
+  default_editor: string | null;
+  theme: string;
+  git_user_name: string | null;
+  git_user_email: string | null;
+}
+
+export interface UserConfigUpdate {
+  default_editor?: string;
+  theme?: string;
+  git_user_name?: string;
+  git_user_email?: string;
+}
+
+export const getUserConfig = async (): Promise<UserConfig> => {
+  const response = await apiClient.get<UserConfig>("/users/me/config");
+  return response.data;
+};
+
+export const updateUserConfig = async (data: UserConfigUpdate): Promise<UserConfig> => {
+  const response = await apiClient.patch<UserConfig>("/users/me/config", data);
+  return response.data;
+};