feat(15-03): rewrite login page and auth hooks for OIDC

- Login page redirects to Logto instead of showing credential form
- AuthState uses string id (Logto sub claim) instead of number
- Remove useLogin, useSetup, useChangePassword hooks
- useLogout redirects to /logout (server-side OIDC logout)
- Remove ChangePasswordSection from settings page
- Update UserMenu to use new useLogout API
- Settings page shows API keys section when authenticated

src/client/components/UserMenu.tsx

@@ -43,7 +43,7 @@ export function UserMenu() {
 						type="button"
 						onClick={() => {
 							setOpen(false);
-							logout.mutate();
+							logout();
 						}}
 						className="flex items-center gap-2 w-full px-3 py-2 text-sm text-gray-700 hover:bg-gray-50 transition-colors"
 					>