feat: 添加视觉设计语言系统并重构认证页面UI

- 新增 visual_design_language.md 设计规范文档
- 新增 auth 设计 tokens (authBackground, authCard, authInput, feedback 系列等)
- 重构登录/注册/验证码/重置密码页面为新设计系统
- 新增 AuthHeroHeader, AuthSurfaceCard, AuthSection, AuthField, PasswordField 组件
- 重构 AppBanner 和 Toast 支持多类型配置 (info/success/warning/error)
- 后端 AgentScope: 重整 schemas/prompts/tools 作用域, 新增协议文档
- 更新 AGENTS.md 集成视觉设计语言约束

apps/lib/features/messages/ui/widgets/calendar_message_card.dart

@@ -1,5 +1,3 @@
-import 'dart:convert';
-
 import 'package:flutter/material.dart';
 
 import '../../../../core/theme/design_tokens.dart';
@@ -19,13 +17,8 @@ class CalendarInviteCard extends StatelessWidget {
   });
 
   String? get eventTitle {
-    if (message.content == null) return null;
-    try {
-      final data = jsonDecode(message.content!) as Map<String, dynamic>;
-      return data['title'] as String?;
-    } catch (_) {
-      return null;
-    }
+    final data = message.content;
+    return data?['title'] as String?;
   }
 
   @override
@@ -101,13 +94,8 @@ class CalendarUpdateCard extends StatelessWidget {
   const CalendarUpdateCard({super.key, required this.message, this.onTap});
 
   String? get eventTitle {
-    if (message.content == null) return null;
-    try {
-      final data = jsonDecode(message.content!) as Map<String, dynamic>;
-      return data['title'] as String?;
-    } catch (_) {
-      return null;
-    }
+    final data = message.content;
+    return data?['title'] as String?;
   }
 
   @override
@@ -190,13 +178,8 @@ class CalendarDeleteCard extends StatelessWidget {
   const CalendarDeleteCard({super.key, required this.message});
 
   String? get eventTitle {
-    if (message.content == null) return null;
-    try {
-      final data = jsonDecode(message.content!) as Map<String, dynamic>;
-      return data['title'] as String?;
-    } catch (_) {
-      return null;
-    }
+    final data = message.content;
+    return data?['title'] as String?;
   }
 
   @override