fix(mobile): move _startDownload to didChangeDependencies to avoid initState context crash

context.t() (Localizations) cannot be called during initState() — inherited
widgets are not yet mounted. When download fails instantly (e.g. no network),
setState with context.t() inside _startDownload fired synchronously within
initState, causing:
  dependOnInheritedWidgetOfExactType called before initState() completed

Fix: add _downloadStarted guard flag; call _startDownload() from
didChangeDependencies() instead of initState(), where context is fully ready.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/genex-mobile/lib/core/updater/channels/self_hosted_updater.dart

@@ -246,19 +246,26 @@ class _DownloadProgressDialogState extends State<_DownloadProgressDialog> {
   bool _hasError = false;
   bool _downloadCompleted = false;
   File? _downloadedApkFile;
+  // 防止 didChangeDependencies 多次触发时重复启动下载
+  bool _downloadStarted = false;
 
   @override
   void initState() {
     super.initState();
-    _startDownload();
+    // 不在 initState 里调用 context.t()，需要等 inherited widget 挂载完成
   }
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
+    // didChangeDependencies 是首次可安全使用 context.t() 的时机
     if (_statusText.isEmpty) {
       _statusText = context.t('update.preparing');
     }
+    if (!_downloadStarted) {
+      _downloadStarted = true;
+      _startDownload();
+    }
   }
 
   Future<void> _startDownload() async {