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>

tests/integration/test_main_window.gd

@@ -11,7 +11,7 @@ func test_startup_loads_soundtracks_and_preloads_a_song() -> void:
 
 
 func test_low_played_mode_fetches_from_low_played_endpoint() -> void:
-	Settings.use_low_played_mode = true
+	Settings.random_mode = Settings.RandomMode.LOW_PLAYED
 	_server.respond_bytes(
 			"/music/rand/low", FileAccess.get_file_as_bytes("res://tests/data/short.mp3"))