refactor: 移除前端 Mock API，新增共享组件，优化认证流程

- 删除 mock_api_client、mock_calendar_service、mock_history_service
- 新增 fixed_length_code_input、link_button、message_composer 共享组件
- 优化登录/注册/密码重置页面使用新组件
- 简化 injection.dart 移除 mock 分支
- 更新 env.dart 配置（BACKEND_URL 替换 API_URL）
- 后端 agentscope 工具和测试更新
- 重构 AGENTS.md 文档结构
- 新增 deploy/ 目录和 protocol 文档

apps/lib/shared/widgets/page_header.dart

@@ -1,5 +1,7 @@
 import 'package:flutter/material.dart';
 
+import '../../core/theme/design_tokens.dart';
+
 class PageHeader extends StatelessWidget {
   final Widget? leading;
   final Widget? trailing;
@@ -32,20 +34,23 @@ class BackButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    return GestureDetector(
-      onTap: onPressed ?? () => Navigator.of(context).pop(),
-      child: Container(
-        width: 36,
-        height: 36,
-        decoration: BoxDecoration(
-          color: const Color(0xFFF8FAFF),
-          borderRadius: BorderRadius.circular(18),
-          border: Border.all(color: const Color(0xFFDEE7F6)),
+    return SizedBox(
+      width: AppSpacing.xxl * 2,
+      height: AppSpacing.xxl * 2,
+      child: TextButton(
+        onPressed: onPressed ?? () => Navigator.of(context).pop(),
+        style: TextButton.styleFrom(
+          padding: const EdgeInsets.all(AppSpacing.none),
+          backgroundColor: AppColors.surfaceTertiary,
+          shape: RoundedRectangleBorder(
+            borderRadius: BorderRadius.circular(AppRadius.full),
+            side: const BorderSide(color: AppColors.borderTertiary),
+          ),
         ),
         child: const Icon(
           Icons.chevron_left,
-          size: 18,
-          color: Color(0xFF334155),
+          size: AppSpacing.lg + AppSpacing.xs,
+          color: AppColors.slate700,
         ),
       ),
     );