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>

data/song_object.gd

@@ -17,6 +17,9 @@ var is_playing: bool = false
 var song_path: String = ""
 ## The song itself, only when it could not be saved to a file.
 var song: AudioStream
+## Where the part played with Short songs starts, in seconds, or -1 before it is picked.
+## Picked once, so restarting and repeating play the same part.
+var short_start: float = -1.0
 
 
 func get_soundtrack_title() -> String: