feat: 新增用户自助注册功能

后端新增 POST /api/v1/public/register 接口，支持门店自助注册并自动生成门店编码（S000001 格式）；shops 表加 description 字段。营销站新增 /register/ 注册页，含门店信息和管理员账号表单，注册成功后展示门店编码和登录提示。Flutter 登录页底部加「前往官网注册」跳转链接。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/lib/screens/auth/login_screen.dart

@@ -3,6 +3,7 @@ import 'dart:async';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+import 'package:url_launcher/url_launcher.dart';
 import '../../core/auth/auth_state.dart';
 import '../../core/theme/app_theme.dart';
 import '../../core/storage/login_history.dart';
@@ -484,6 +485,20 @@ class _LoginScreenState extends ConsumerState<LoginScreen> {
                                             letterSpacing: 2)),
                               ),
                             ),
+                            const SizedBox(height: 12),
+                            TextButton(
+                              onPressed: () => launchUrl(
+                                Uri.parse('https://jiu.51yanmei.com/register/'),
+                                mode: LaunchMode.externalApplication,
+                              ),
+                              child: const Text(
+                                '还没有门店账号？前往官网注册',
+                                style: TextStyle(
+                                  fontSize: 13,
+                                  color: AppTheme.textSecondary,
+                                ),
+                              ),
+                            ),
                           ],
                         ),
                       ),