Add public Android APK download

frontend/src/index.css

@@ -391,6 +391,9 @@ select {
 .auth-link { text-align: center; margin-top: 20px; font-size: 13px; color: var(--gray-500); }
 .auth-link a { color: var(--primary); font-weight: 600; text-decoration: none; }
 .auth-link a:hover { text-decoration: underline; }
+.auth-download-btn {
+  justify-content: center;
+}
 
 /* ─── Form fields ────────────────────────────────────────────── */
 .field { display: flex; flex-direction: column; gap: 5px; }

frontend/src/pages/auth/LoginPage.jsx

@@ -9,6 +9,8 @@ export default function LoginPage() {
   const navigate = useNavigate();
   const [form, setForm] = useState({ email: '', password: '' });
   const [loading, setLoading] = useState(false);
+  const apiBaseUrl = (import.meta.env.VITE_API_URL || 'http://localhost:4000').replace(/\/$/, '');
+  const androidApkUrl = `${apiBaseUrl}/downloads/android-apk`;
 
   const handleChange = (e) => setForm((f) => ({ ...f, [e.target.name]: e.target.value }));
 
@@ -54,6 +56,9 @@ export default function LoginPage() {
           <button type="submit" className="btn btn-primary btn-full" disabled={loading}>
             {loading ? 'Ingresando...' : 'Iniciar sesión'}
           </button>
+          <a className="btn btn-secondary btn-full auth-download-btn" href={androidApkUrl}>
+            Descargar app Android
+          </a>
         </form>
         <p className="auth-link">
           ¿No tienes cuenta? <Link to="/register">Registrarse</Link>