fix: DNS 用 sing-box 1.12+ 新格式 + 引导仅首次显示

- clientconfig/mockserver:客户端配置 DNS 从 legacy address 串格式改为
  type+server 新格式(sing-box 1.13 对 legacy DNS 直接 FATAL,导致连不上)
- 引导页只首次:TokenStore 加 markOnboarded/isOnboarded;main.dart 登录后
  若已引导过直接进主界面,不再每次都弹引导

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/services/token_store.dart

@@ -13,6 +13,7 @@ class TokenStore {
 
   static const _kAccess = 'pangolin_access_token';
   static const _kRefresh = 'pangolin_refresh_token';
+  static const _kOnboarded = 'pangolin_onboarded';
 
   Future<void> saveTokens({
     required String access,
@@ -29,4 +30,8 @@ class TokenStore {
     await _storage.delete(key: _kAccess);
     await _storage.delete(key: _kRefresh);
   }
+
+  // 首次引导标记:引导页只在第一次看(看完置位,后续登录直接进主界面)。
+  Future<void> markOnboarded() => _storage.write(key: _kOnboarded, value: '1');
+  Future<bool> isOnboarded() async => (await _storage.read(key: _kOnboarded)) == '1';
 }