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/shared/widgets/link_button.dart

@@ -20,28 +20,33 @@ class LinkButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final effectiveColor = foregroundColor ?? AppColors.slate500;
+    final color = enabled
+        ? (foregroundColor ?? AppColors.authLinkText)
+        : AppColors.slate300;
 
-    return SizedBox(
-      height: 44,
-      child: TextButton(
-        onPressed: enabled ? onTap : null,
-        style: TextButton.styleFrom(
-          foregroundColor: enabled ? effectiveColor : AppColors.slate300,
-          padding: const EdgeInsets.symmetric(
-            horizontal: AppSpacing.md,
-            vertical: AppSpacing.sm,
-          ),
-          shape: RoundedRectangleBorder(
-            borderRadius: BorderRadius.circular(AppRadius.sm),
-          ),
+    return TextButton(
+      onPressed: enabled ? onTap : null,
+      style: TextButton.styleFrom(
+        minimumSize: const Size(0, 40),
+        tapTargetSize: MaterialTapTargetSize.shrinkWrap,
+        foregroundColor: color,
+        padding: const EdgeInsets.symmetric(
+          horizontal: AppSpacing.md,
+          vertical: AppSpacing.sm,
         ),
-        child: Text(
-          text,
-          style: const TextStyle(fontSize: 14, fontWeight: FontWeight.w500),
-          textAlign: textAlign,
+        shape: RoundedRectangleBorder(
+          borderRadius: BorderRadius.circular(AppRadius.full),
         ),
       ),
+      child: Text(
+        text,
+        style: TextStyle(
+          fontSize: 14,
+          fontWeight: FontWeight.w600,
+          color: color,
+        ),
+        textAlign: textAlign,
+      ),
     );
   }
 }