fix: use _is_stopped flag for queue start check

_active_download tracks single download; queue uses _is_stopped. Wrong flag
caused false 'already active' errors when queue idle.

src/server/services/download_service.py

@@ -828,8 +828,8 @@ class DownloadService:
             # Initialize queue progress tracking if not already done
             await self._init_queue_progress()
             
-            # Check if download already active
-            if self._active_download:
+            # Check if queue is already running
+            if not self._is_stopped:
                 raise DownloadServiceError(
                     "Queue processing is already active"
                 )