feat: hide Google sign-in in embedded WebView (DApps browser)

Google blocks OAuth in WebViews (disallowed_useragent policy).
Detect WebView and hide the Google button, users in DApps browser
use email/password instead. Google sign-in still works in real
browsers (Chrome, Safari, etc).

web/src/pages/Login.tsx

@@ -24,6 +24,11 @@ const Login: React.FC = () => {
   const [error, setError] = useState('');
   const [loading, setLoading] = useState(false);
 
+  // Detect embedded WebView (DApps browser) - hide Google OAuth there
+  const isWebView = /wv|WebView/i.test(navigator.userAgent) ||
+    (/(iPhone|iPod|iPad).*AppleWebKit(?!.*Safari)/i.test(navigator.userAgent)) ||
+    (/Android.*Version\/[\d.]+.*Chrome\/[\d.]+ Mobile/i.test(navigator.userAgent) && !/Chrome\/[\d.]+ Mobile Safari/i.test(navigator.userAgent));
+
   const [loginData, setLoginData] = useState({
     email: '',
     password: ''
@@ -382,6 +387,7 @@ const Login: React.FC = () => {
           </div>
           
           <div className="space-y-3">
+            {!isWebView && (
               <Button
                 variant="outline"
                 className="w-full border-gray-700 bg-gray-800 hover:bg-gray-700 text-white"
@@ -396,6 +402,7 @@ const Login: React.FC = () => {
                 </svg>
                 {t('login.googleSignIn', 'Continue with Google')}
               </Button>
+            )}
 
             <Button
               variant="outline"