统一弹窗风格：Material Design 3 规范，消除颜色不一致

- 所有 AlertDialog 替换为 ModernDialog
- ConfirmDialog/AssetDialogs 去掉 GlassPanel，统一 surfaceContainer 背景
- 按钮统一 FilledButton + TextButton
- 修复 import 路径

flutter_monisuo/lib/ui/pages/auth/login_page.dart

@@ -7,6 +7,7 @@ import '../../../core/theme/app_theme.dart';
 import '../../../core/theme/app_theme_extension.dart';
 import '../../../providers/auth_provider.dart';
 import '../../components/material_input.dart';
+import '../../shared/modern_dialog.dart';
 import '../main/main_page.dart';
 import 'register_page.dart';
 
@@ -296,18 +297,10 @@ class _LoginPageState extends State<LoginPage> {
   }
 
   void _showErrorDialog(String message) {
-    showDialog(
+    ModernDialog.info(
       context: context,
-      builder: (context) => AlertDialog(
-        title: const Text('登錄失敗'),
-        content: Text(message),
-        actions: [
-          TextButton(
-            onPressed: () => Navigator.of(context).pop(),
-            child: const Text('確定'),
-          ),
-        ],
-      ),
+      title: '登錄失敗',
+      description: message,
     );
   }
 }