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.

[damontecres]

It would be better to use the built-in KeyEvent repeat count. Each repeat is roughly 250-300ms, so you could wait until repeatCount==2.

[jbatesy]

Haven't tested this in depth, will update you when it's ready for another look.

[jbatesy]

@damontecres I cherry picked the commits in my branch over the upcoming release branch (locally) & it works well, changes also work on main branch. Please review when you have the time.

[damontecres]

Looks good to me!