[Feature Request] Store all download information as a file

[the-liquid-metal]

• Since ytdlp-interface saves its configuration as ".json", it would be nice if the saved file is saved as ".json" as well.
• In the saved file, Download entry is designed to be as flat as possible (not a complicated structure), so that users can transform, manipulate, add, or delete manually easily.
• Contrary to the current workflow, where completed downloads are deleted at exit, all downloaded items are saved permanently. Users can still delete items as of now.
• Contrary to the current state, where undownloaded items are refetched repeatedly at startup, all items are only fetched once when added to the list. Users can still refetched them when desired, all at once or one by one. This state can save time when there are dozens/hundreds of items that have not been downloaded.
• Before exiting the app, there should be a message whether user wants to save the list or not.
• When saving for the first time, there should be a window to collect info: filename and its location, and where to store the thumbnails. The app automatically suggest thumbnails folder name and its location: ".assets" located at the same folder as the saved file. User can changes its name and location. If there are more than one the saved file in a folder, those files can use the same ".assets" folder.
• To prevent conflict, the thumbnail filename is a simple hashed string or UUID, and user cannot change it via the app.
• If the thumbnail file is lost, the app only displays a missing file message. Users can fix this problem by refetching or placing a new image manually.
• The app does not care the existance of downloaded files, and not try to recover if it is lost. There is no missing file message. Users can fix this problem by redownloading or placing a new video/song manually.