fix: changelog default value and referrals map syntax

- admin-service: provide default empty string for optional changelog
- profile page: fix map literal syntax for referrals list

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/admin-service/src/api/controllers/version.controller.ts

@@ -348,7 +348,7 @@ export class VersionController {
       dto.platform,
       file.buffer,
       file.originalname,
-      dto.changelog,
+      dto.changelog ?? '',
       dto.isForceUpdate ?? false,
       'admin', // TODO: Get from JWT token
       dto.versionCode,

frontend/mobile-app/lib/features/profile/presentation/pages/profile_page.dart

@@ -230,7 +230,7 @@ class _ProfilePageState extends ConsumerState<ProfilePage> {
           _leaderboardRank = referralInfo.leaderboardRank;
 
           // 转换直推列表格式
-          _referrals = directReferrals.referrals.map((r) => {
+          _referrals = directReferrals.referrals.map((r) => <String, dynamic>{
             'serial': r.userId,
             'personal': 0, // API暂未返回个人认种量
             'team': r.teamCount,