fix(oauth): set GOOGLE_REDIRECT_URI in docker-compose; guard missing credentials

- Add APP_URL and GOOGLE_REDIRECT_URI=https://scoreodonto.com/api/oauth2callback
  to docker-compose backend environment
- /api/auth/google/url returns 503 with clear message when CLIENT_ID/SECRET unset
- GoogleConnectButton shows alert instead of silently breaking on API error

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/components/GoogleConnectButton.tsx

@@ -14,7 +14,13 @@ export const GoogleConnectButton: React.FC<GoogleConnectButtonProps> = ({ ownerI
         setIsLoading(true);
         try {
             const response = await fetch(`/api/auth/google/url?dentistId=${ownerId}`);
-            const { url } = await response.json();
+            const data = await response.json();
+            if (!response.ok || data.error) {
+                alert(`Erro: ${data.error || 'Falha ao obter URL de autenticação'}`);
+                setIsLoading(false);
+                return;
+            }
+            const { url } = data;
 
             // Abrir em popup para manter o contexto
             const width = 600;