Add batch add and folder scanning functions to local files

### Description

I really like this software and am very accustomed to its interface, so I also want to use this software to play my local audio files.

Under normal circumstances, users would definitely want to batch add local files or directly scan the selected folders, as it would be both convenient and fast.
However, when using this software to add files, I found that the current local file addition feature can only add them one by one, which seems very cumbersome and very inconvenient for users.

Therefore, I hope the developers can add batch processing and folder scanning features to the local file addition, allowing users to more conveniently and elegantly use this software to listen to local audio files. Thank you~

### Screenshots or screen recording

###

### Did you search for existing list?

- [x] I have searched for existing issues.

No file, test, or entry point is named. Start by tracing the existing local file addition flow in the Android app and determine how it currently selects individual files; done means users can add multiple local audio files and scan selected folders through the existing interface.

Written by the indexing model from the issue text.