fix: guard processQueue against concurrent invocations, fix stale process kill reference

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.ts

@@ -3945,7 +3945,9 @@ ipcMain.handle('start-download', async () => {
     saveQueue(downloadQueue);
     emitQueueUpdated();
 
-    processQueue();
+    if (!isDownloading) {
+        void processQueue();
+    }
     return true;
 });