Add loading on thumbnail in playlist tab

[koonchaya]

while the spectrogram loading in thumbnail, it should show loading indicator.

Screen.Recording.2569-01-05.at.18.59.41.mov

[koonchaya]

@rassokhina-e It should show loading instead of text in this case, selecting another recording from thumbnail.
https://github.com/user-attachments/assets/624e7b7b-7104-4e1e-af7c-7ee3c3ebfc3a