feat(flutter): 接入分类记录收藏与下载授权

lib/core/widgets/auth_gate.dart

@@ -9,9 +9,7 @@ import '../theme/app_colors.dart';
 /// 打开登录 / 注册页（沿用现有 Navigator 风格，不引路由库）
 Future<void> openAuthPage(BuildContext context, {bool signUpMode = false}) {
   return Navigator.of(context).push(
-    MaterialPageRoute<void>(
-      builder: (_) => AuthPage(signUpMode: signUpMode),
-    ),
+    MaterialPageRoute<void>(builder: (_) => AuthPage(signUpMode: signUpMode)),
   );
 }
 
@@ -81,3 +79,44 @@ Future<bool> ensureSignedIn(BuildContext context) async {
   }
   return false;
 }
+
+/// 页面级登录引导，用于记录、收藏和下载等必须登录的独立页面。
+class AuthGatePanel extends StatelessWidget {
+  const AuthGatePanel({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context)!;
+    return Center(
+      key: const Key('auth-gate-panel'),
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 32),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Text(
+              l10n.authGateTitle,
+              textAlign: TextAlign.center,
+              style: const TextStyle(
+                color: Colors.white,
+                fontSize: 18,
+                fontWeight: FontWeight.w800,
+              ),
+            ),
+            const SizedBox(height: 8),
+            Text(
+              l10n.authGateMessage,
+              textAlign: TextAlign.center,
+              style: const TextStyle(color: AppColors.muted, fontSize: 13),
+            ),
+            const SizedBox(height: 18),
+            FilledButton(
+              onPressed: () => openAuthPage(context),
+              child: Text(l10n.authGateAction),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}