Fix/playlist sort direction

[adrielGGmotion]

Summary

Fixes #2779 by separating the playlist sort direction preference between the library screen and the "Add to Playlist" popup.

Problem

The library screen and "Add to Playlist" popup were sharing the same preference key playlistSortDescending. This meant:

• Changing sort direction in the popup would unexpectedly change the library view
• Users had to re-sort their library every time they added a song to a playlist

Solution

Changed line 154 in PreferenceKeys.kt to use a unique preference key:

• Before: booleanPreferencesKey("playlistSortDescending") (shared)
• After: booleanPreferencesKey("addToPlaylistSortDescending") (unique)

Testing

1. Go to Library → Playlists, sort by "Name" (ascending)
2. Pick any song → "Add to Playlist"
3. In popup, change sort to "Creation Date" (descending)
4. Close popup and return to Library
5. Expected: Library still shows "Name" (ascending) - independent from popup

Checklist

• Bug fix (non-breaking change)
• Fixes Playlist sort direction linked between library screen and "Add to Playlist" popup #2779

[adrielGGmotion]

That's my favorite pull request because that was the easier one

[diam0ndkiller]

Thanks a ton! It's those simple fixes that really show the benefits of Open Source :)