Add Google OAuth admin login; fix header comment; both auth methods active

admin/auth/google/login.php

@@ -0,0 +1,30 @@
+<?php
+/**
+ * Redirect to Google OAuth — admin login entry point
+ */
+require_once __DIR__ . '/../../../includes/auth.php';
+
+if (AdminAuth::isLoggedIn()) {
+    header('Location: /admin/');
+    exit;
+}
+
+if (!defined('GOOGLE_CLIENT_ID') || !GOOGLE_CLIENT_ID) {
+    die('Google OAuth is not configured. Set GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET in config.');
+}
+
+$state = bin2hex(random_bytes(16));
+$_SESSION['google_oauth_state'] = $state;
+
+$params = http_build_query([
+    'client_id'     => GOOGLE_CLIENT_ID,
+    'redirect_uri'  => GOOGLE_REDIRECT_URI,
+    'response_type' => 'code',
+    'scope'         => 'openid email profile',
+    'state'         => $state,
+    'access_type'   => 'online',
+    'prompt'        => 'select_account',
+]);
+
+header('Location: https://accounts.google.com/o/oauth2/v2/auth?' . $params);
+exit;