fix(mining-app): 更新弹窗点击"立即更新"无反应 — 进度流订阅竞态修复

问题根因:
SelfHostedUpdater._listenToProgress() 在 initState 和 _startDownload 中
订阅 UpdateService.downloadProgressStream，但该 getter 返回
_downloadManager?.progressStream —— _downloadManager 在 downloadUpdate()
调用前为 null，?.listen 无操作。downloadUpdate() 创建新 _downloadManager
后，其 progressStream 无人监听，进度事件全部丢失，UI 无任何反馈。

修复:
1. UpdateService 新增持久化广播流 _downloadProgressController，
   downloadUpdate() 创建新 DownloadManager 后将其进度转发到此流
2. downloadProgressStream getter 改为返回持久化流（非 nullable）
3. SelfHostedUpdater 在 initState 中一次性订阅持久化流，
   dispose 时取消订阅，_startDownload 不再重复订阅

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

frontend/mining-app/lib/core/updater/channels/self_hosted_updater.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:io';
 import 'package:flutter/material.dart';
 import '../models/version_info.dart';
@@ -48,15 +49,13 @@ class _SelfHostedUpdaterState extends State<SelfHostedUpdater> {
   DownloadProgress _progress = DownloadProgress.initial();
   File? _downloadedFile;
   bool _isDownloading = false;
+  StreamSubscription<DownloadProgress>? _progressSubscription;
 
   @override
   void initState() {
     super.initState();
-    _listenToProgress();
+    // 订阅持久化广播流，无论何时开始下载都能收到进度
-  }
+    _progressSubscription = UpdateService.instance.downloadProgressStream.listen((progress) {
-
-  void _listenToProgress() {
-    UpdateService.instance.downloadProgressStream?.listen((progress) {
       if (mounted) {
         setState(() {
           _progress = progress;
@@ -65,6 +64,12 @@ class _SelfHostedUpdaterState extends State<SelfHostedUpdater> {
     });
   }
 
+  @override
+  void dispose() {
+    _progressSubscription?.cancel();
+    super.dispose();
+  }
+
   Future<void> _startDownload() async {
     setState(() {
       _isDownloading = true;
@@ -73,8 +78,6 @@ class _SelfHostedUpdaterState extends State<SelfHostedUpdater> {
       );
     });
 
-    _listenToProgress();
-
     final file = await UpdateService.instance.downloadUpdate(widget.versionInfo);
 
     if (mounted) {

frontend/mining-app/lib/core/updater/update_service.dart

@@ -51,6 +51,10 @@ class UpdateService {
   final _updateAvailableController = StreamController<VersionInfo?>.broadcast();
   Stream<VersionInfo?> get updateAvailableStream => _updateAvailableController.stream;
 
+  /// 持久化的下载进度流（跨 DownloadManager 实例保持订阅有效）
+  final _downloadProgressController = StreamController<DownloadProgress>.broadcast();
+  StreamSubscription? _downloadProgressSubscription;
+
   VersionInfo? _latestVersion;
   VersionInfo? get latestVersion => _latestVersion;
 
@@ -126,14 +130,20 @@ class UpdateService {
 
   /// 下载更新
   Future<File?> downloadUpdate(VersionInfo versionInfo) async {
+    // 取消旧的进度转发订阅
+    await _downloadProgressSubscription?.cancel();
     _downloadManager?.dispose();
     _downloadManager = DownloadManager(versionInfo: versionInfo);
+    // 将新 DownloadManager 的进度转发到持久化广播流
+    _downloadProgressSubscription = _downloadManager!.progressStream.listen(
+      (progress) => _downloadProgressController.add(progress),
+    );
     return await _downloadManager!.startDownload();
   }
 
-  /// 获取下载进度流
+  /// 获取下载进度流（持久化广播流，订阅后可跨下载任务接收进度）
-  Stream<DownloadProgress>? get downloadProgressStream =>
+  Stream<DownloadProgress> get downloadProgressStream =>
-      _downloadManager?.progressStream;
+      _downloadProgressController.stream;
 
   /// 取消下载
   void cancelDownload() {
@@ -166,7 +176,9 @@ class UpdateService {
   }
 
   void dispose() {
+    _downloadProgressSubscription?.cancel();
     _downloadManager?.dispose();
+    _downloadProgressController.close();
     _updateAvailableController.close();
   }
 }