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/auth/ui/widgets/password_field.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+
+import '../../../../core/theme/design_tokens.dart';
+import 'auth_field.dart';
+
+class PasswordField extends StatefulWidget {
+  const PasswordField({
+    super.key,
+    required this.controller,
+    required this.label,
+    required this.hint,
+    this.onChanged,
+  });
+
+  final TextEditingController controller;
+  final String label;
+  final String hint;
+  final ValueChanged<String>? onChanged;
+
+  @override
+  State<PasswordField> createState() => _PasswordFieldState();
+}
+
+class _PasswordFieldState extends State<PasswordField> {
+  bool _obscured = true;
+
+  void _toggleVisibility() {
+    setState(() {
+      _obscured = !_obscured;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return AuthField(
+      label: widget.label,
+      hint: widget.hint,
+      controller: widget.controller,
+      obscureText: _obscured,
+      onChanged: widget.onChanged,
+      suffixIcon: IconButton(
+        onPressed: _toggleVisibility,
+        tooltip: _obscured ? '显示密码' : '隐藏密码',
+        icon: Icon(
+          _obscured ? Icons.visibility_off_rounded : Icons.visibility_rounded,
+          color: AppColors.authInputIcon,
+        ),
+      ),
+    );
+  }
+}