feat(mining-app): 启动和切回前台自动检测升级

MainShell 从 StatelessWidget 转为 StatefulWidget + WidgetsBindingObserver，
参考 mobile-app HomeShellPage 模式，实现：
- initState 首帧后自动检查更新
- didChangeAppLifecycleState(resumed) 从后台恢复时自动检查
- 90-300秒随机冷却间隔防止频繁请求（static 变量跨 rebuild 保持）
- 延迟3秒启动检查，避免干扰用户操作
- 有更新时弹出 SelfHostedUpdater 对话框（支持强制更新）

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

frontend/mining-app/lib/presentation/widgets/main_shell.dart

@@ -1,19 +1,82 @@
+import 'dart:math';
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import '../../core/router/routes.dart';
 import '../../core/constants/app_colors.dart';
+import '../../core/updater/update_service.dart';
+import '../../core/updater/channels/self_hosted_updater.dart';
 
-class MainShell extends StatelessWidget {
+class MainShell extends StatefulWidget {
   final Widget child;
 
   const MainShell({super.key, required this.child});
 
+  @override
+  State<MainShell> createState() => _MainShellState();
+}
+
+class _MainShellState extends State<MainShell> with WidgetsBindingObserver {
+  /// 下次允许检查更新的时间（static 防止 widget rebuild 重置）
+  static DateTime? _nextCheckAllowedTime;
+
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addObserver(this);
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      _checkForUpdateIfNeeded();
+    });
+  }
+
+  @override
+  void dispose() {
+    WidgetsBinding.instance.removeObserver(this);
+    super.dispose();
+  }
+
+  @override
+  void didChangeAppLifecycleState(AppLifecycleState state) {
+    if (state == AppLifecycleState.resumed) {
+      _checkForUpdateIfNeeded();
+    }
+  }
+
+  Future<void> _checkForUpdateIfNeeded() async {
+    final now = DateTime.now();
+
+    // 如果从未检查过，或者已过冷却时间，则检查
+    if (_nextCheckAllowedTime == null || now.isAfter(_nextCheckAllowedTime!)) {
+      // 设置下次允许检查时间（90-300秒随机间隔）
+      final randomSeconds = 90 + Random().nextInt(211);
+      _nextCheckAllowedTime = now.add(Duration(seconds: randomSeconds));
+
+      // 延迟3秒，避免启动时干扰用户
+      await Future.delayed(const Duration(seconds: 3));
+      if (!mounted) return;
+
+      try {
+        final result = await UpdateService.instance.checkForUpdate();
+        if (!mounted) return;
+
+        if (result.hasUpdate && result.versionInfo != null) {
+          await SelfHostedUpdater.show(
+            context,
+            versionInfo: result.versionInfo!,
+            isForceUpdate: result.versionInfo!.isForceUpdate,
+          );
+        }
+      } catch (e) {
+        debugPrint('[MainShell] 检查更新失败: $e');
+      }
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final isDark = AppColors.isDark(context);
 
     return Scaffold(
-      body: child,
+      body: widget.child,
       bottomNavigationBar: Container(
         decoration: BoxDecoration(
           color: AppColors.cardOf(context),