feat: add task cancellation support

- Add CancelTask function to cancel running/waiting tasks
- Add /api/cancelTask endpoint
- Add cancel button in task list for running/waiting tasks
- Check cancellation status during download and merge operations

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

internal/router/router.go

@@ -28,6 +28,7 @@ func API() *http.ServeMux {
 	router.HandleFunc("/quit", quit)
 	router.HandleFunc("/getPopularVideos", getPopularVideos)
 	router.HandleFunc("/deleteTask", deleteTask)
+	router.HandleFunc("/cancelTask", cancelTask)
 	router.HandleFunc("/getRedirectedLocation", getRedirectedLocation)
 	router.HandleFunc("/downloadVideo", downloadVideo)
 	router.HandleFunc("/getSeasonsArchivesListFirstBvid", getSeasonsArchivesListFirstBvid)