Fix Roku subtitle menu for burned-in tracks

[VX-101]

Changes

Fixes the in-playback subtitle menu state when playback is transcoding with burned-in subtitles.

When burn-in subtitles were enabled, onVideoContentLoaded() marked subtitle tracks as encoded but stopped iterating after finding the selected track. That meant any later subtitle tracks were dropped from fullSubtitleData, so the playback subtitle menu could show an incomplete list.

The player also skipped updating selectedSubtitle during burned-in subtitle playback, so the menu could show None even though Jellyfin had selected and burned in a default subtitle stream.

This change:

• keeps processing the full subtitle list when marking burned-in tracks as encoded
• always updates selectedSubtitle while the observer is temporarily removed
• preserves the existing observer guard so this state update does not trigger an unnecessary reload

Fixes #851.

Testing

• Ran npm run build successfully.
• Sideloaded the generated Roku package.
• Tested with an MKV containing two English PGS subtitle tracks.
• Confirmed the in-playback subtitle menu shows None plus both PGS tracks.
• Confirmed the default burned-in subtitle track is selected correctly and default CC works.

[jimdogx]

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