Show subtitle track info on the item details screen.

[VX-101]

Summary

• Add a subtitle info row below the existing video and audio rows on the item details screen.
• Display the default subtitle stream when one is available, with +N for additional subtitle streams.
• Display No Default when subtitle streams exist and none is marked default.
• Show the Jellyfin default subtitle track as checked in the details Options subtitle list when no subtitle preference has been selected.

Testing

• Ran npm run build successfully.
• Sideloaded the generated Roku package.
• Tested locally with generated media clips covering default, no-default, forced internal text, external SRT, no-subtitle, single-subtitle, and forced-default subtitle cases.
• Compared the Roku subtitle info row with Jellyfin subtitle metadata.
• Verified the details Options subtitle checkmark for default and no-default subtitle cases.

Known issue

• In some cases, Roku can still show a subtitle track that does not match the user’s selected option. This includes cases where None is selected, but a subtitle track is still shown. (see Fix None subtitle playback selection after #854.  #868 for the fix)

[VX-101]

I saw PR #728, but it was opened back on February 9 and still has not been merged. My changes are small and localized, so I believe they can be reviewed and merged independently.
@1hitsong Please take a look. This PR, together with PR #858, resolves issues related to displaying default/selected subtitles. See #868