style(theme): 客户端切换深色视觉风格

lib/core/widgets/auth_gate.dart

@@ -37,7 +37,7 @@ Future<bool> ensureSignedIn(BuildContext context) async {
             Text(
               l10n.authGateTitle,
               style: const TextStyle(
-                color: Colors.white,
+                color: AppColors.onSurface,
                 fontSize: 16,
                 fontWeight: FontWeight.w800,
               ),
@@ -98,7 +98,7 @@ class AuthGatePanel extends StatelessWidget {
               l10n.authGateTitle,
               textAlign: TextAlign.center,
               style: const TextStyle(
-                color: Colors.white,
+                color: AppColors.onSurface,
                 fontSize: 18,
                 fontWeight: FontWeight.w800,
               ),