fix(mobile): 修复序列号类型转换错误

accountSequence 格式已从数字改为字符串 (D + YYMMDD + 5位序号)
- profile_page.dart: `as int?` → `as String?`
- mining_page.dart: `as int?` → `as String?`

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

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

frontend/mobile-app/lib/features/mining/presentation/pages/mining_page.dart

@@ -69,7 +69,7 @@ class _MiningPageState extends ConsumerState<MiningPage> {
       accountService.getLocalAvatarPath(),
     ]);
 
-    final serialNum = results[0] as int?;
+    final serialNum = results[0] as String?;
     final avatarSvg = results[1] as String?;
     final avatarUrl = results[2] as String?;
     final localAvatarPath = results[3] as String?;

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

@@ -165,7 +165,7 @@ class _ProfilePageState extends ConsumerState<ProfilePage> {
     ]);
 
     final username = results[0] as String?;
-    final serialNum = results[1] as int?;
+    final serialNum = results[1] as String?;
     final avatarSvg = results[2] as String?;
     final avatarUrl = results[3] as String?;
     final localAvatarPath = results[4] as String?;