Use remote up/down key for previous/next scene

[jbatesy]

Resolves #760

This pull request enhances the playback navigation experience by enabling Up/Down key navigation for both scene and marker playlists, and introduces "hold to skip" functionality for these keys. It also updates the relevant preference descriptions to reflect these changes.

Playback navigation improvements:

• Added support for using the Up/Down keys for previous/next navigation in both scene and marker playlists, not just marker playlists. (PlaybackPageContent.kt, StashPreference.kt, strings.xml) [1] [2] [3]
• Implemented "hold to skip" functionality: holding Up or Down triggers immediate previous/next media item navigation, while a short press only shows the player controls. (PlaybackPageContent.kt) [1] [2]

User interface and preferences:

• Updated the preference summary and string resources to clarify that Up/Down navigation applies to both scene and marker playlists. (StashPreference.kt, strings.xml) [1] [2].

[jbatesy]

@damontecres This is ready for review when you have the time.