feat: 新人初始礼包购买追踪功能

- 数据库：添加 has_purchased_starter_pack 字段到 register_bonus_claims
- 后端：创建静态配置管理套餐信息，支持按国家/地区区分
- 后端：新增 GET /api/v1/points/packages API 返回可用套餐
- 后端：创建 utils/paths.py 统一路径管理
- 前端：动态获取套餐信息，移除硬编码
- 前端：添加 ProductCode 枚举约束，前后端类型安全
- 配置：Profile 默认国家改为 US（ISO 3166-1 alpha-2）
- 文档：更新协议文档说明新 API 和字段

apps/lib/features/settings/data/apis/profile_api.dart

@@ -111,7 +111,7 @@ class ProfileApi {
             aiLanguage: (preferencesRaw['ai_language'] as String?) ?? 'zh-CN',
             timezone:
                 (preferencesRaw['timezone'] as String?) ?? 'Asia/Shanghai',
-            country: (preferencesRaw['country'] as String?) ?? 'CN',
+            country: (preferencesRaw['country'] as String?) ?? 'US',
           )
         : const PreferenceSettings();
 

apps/lib/features/settings/data/models/profile_settings.dart

@@ -16,7 +16,7 @@ class PreferenceSettings {
     this.interfaceLanguage = 'zh-CN',
     this.aiLanguage = 'zh-CN',
     this.timezone = 'Asia/Shanghai',
-    this.country = 'CN',
+    this.country = 'US',
   });
 
   final String interfaceLanguage;