feat: improve subtitle picking logic - persist subtitle language selection

[huynhsontung]

When users select a subtitle, they will likely want the same subtitle language for the next media, similar to how YouTube operates. Since there is no standard method to detect the language of a subtitle track, we attempt to retain and compare both the language tag and the language label of the track. If both methods fail, we will select based on the preferred system language.

The logic is as follows:

• When the user opens the subtitle track picker and selects a subtitle, the app tries to guess what language the subtitle track is and remembers that information.
• When opening another media file, if the persistent value is not empty
  • Select the first subtitle track if there is only one.
  • Try to find the first subtitle track in the same language if there is more than one.
• Reset the persistent value to empty when the user opens the track picker and disable the subtitle.

Closes #515