debug(mobile): 下载失败诊断日志

添加详细 debugPrint，覆盖：
- URL 检查失败原因
- 实际下载 URL 和保存路径
- HTTP 响应状态码和 content-length/content-range
- DioException 类型、消息、HTTP 状态码
- SHA-256 校验结果（预期值 vs 实际值）
- 文件大小

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

frontend/genex-mobile/lib/core/updater/download_manager.dart

@@ -34,7 +34,7 @@ class DownloadManager {
   }) async {
     try {
       if (!url.startsWith('https://')) {
-        debugPrint('Download URL must use HTTPS');
+        debugPrint('[DownloadManager] ERROR: URL must use HTTPS, got: $url');
         _status = DownloadStatus.failed;
         return null;
       }
@@ -51,12 +51,16 @@ class DownloadManager {
       int downloadedBytes = 0;
       if (await tempFile.exists()) {
         downloadedBytes = await tempFile.length();
+        debugPrint('[DownloadManager] Resume from $downloadedBytes bytes');
       }
 
       if (await file.exists()) {
         await file.delete();
       }
 
+      debugPrint('[DownloadManager] Starting download: $url');
+      debugPrint('[DownloadManager] Save path: $savePath');
+
       final response = await _dio.get<ResponseBody>(
         url,
         cancelToken: _cancelToken,
@@ -70,9 +74,11 @@ class DownloadManager {
         ),
       );
 
+      debugPrint('[DownloadManager] Response status: ${response.statusCode}');
       int totalBytes = 0;
       final contentLength = response.headers.value('content-length');
       final contentRange = response.headers.value('content-range');
+      debugPrint('[DownloadManager] content-length=$contentLength content-range=$contentRange');
 
       if (contentRange != null) {
         final match = RegExp(r'bytes \d+-\d+/(\d+)').firstMatch(contentRange);
@@ -104,25 +110,29 @@ class DownloadManager {
 
       _status = DownloadStatus.verifying;
 
+      debugPrint('[DownloadManager] Verifying SHA-256, file size=${await file.length()}');
       final isValid = await _verifySha256(file, sha256Expected);
       if (!isValid) {
-        debugPrint('SHA-256 verification failed');
+        debugPrint('[DownloadManager] SHA-256 FAILED, expected=$sha256Expected');
         await file.delete();
         _status = DownloadStatus.failed;
         return null;
       }
+      debugPrint('[DownloadManager] SHA-256 OK');
 
       _status = DownloadStatus.completed;
       return file;
     } on DioException catch (e) {
       if (e.type == DioExceptionType.cancel) {
+        debugPrint('[DownloadManager] Cancelled by user');
         _status = DownloadStatus.cancelled;
       } else {
+        debugPrint('[DownloadManager] DioException type=${e.type} msg=${e.message} status=${e.response?.statusCode}');
         _status = DownloadStatus.failed;
       }
       return null;
-    } catch (e) {
-      debugPrint('Download failed: $e');
+    } catch (e, stack) {
+      debugPrint('[DownloadManager] Unexpected error: $e\n$stack');
       _status = DownloadStatus.failed;
       return null;
     }