Add Short songs and a choice of how random songs are picked (#30, #51)

Short songs (#30): a quiz settings switch and a Seconds per song row (30 by
default, 10-120 in steps of 5), saved with the quiz settings. Only part of
each quiz song plays, from a random point 10-60% into it, picked once per song
so restart and repeat play the same part. The progress bar, time and seeking
stay within the part, a hidden beginning counts inside it, and its end counts
as the song ending (repeat, stop or go to the next song as set).

Random songs (#51): the low-played switch in Settings is now a Normal / Low
played / Classic choice, stored as Settings.random_mode and not saved.
fetch_random_song(mode) picks /music/rand, /music/rand/low or
/music/rand/classic.

Closes #30
Closes #51

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

quiz/song_loader.gd

@@ -122,7 +122,7 @@ func _download_random_song(new_song_object: SongObject) -> void:
 			return
 		Playlist.store_song(new_song_object, body)
 		_add_song_to_queue(new_song_object)
-	MusicServerClient.fetch_random_song(Settings.use_low_played_mode, song_downloaded,
+	MusicServerClient.fetch_random_song(Settings.random_mode, song_downloaded,
 			_retry_later.bind(_download_random_song.bind(new_song_object)))