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_quiz_settings_card.gd

@@ -177,3 +177,33 @@ func test_changes_are_saved_at_once() -> void:
 	assert_int(saved.load(Settings.saved_settings_path())).is_equal(OK)
 	assert_bool(saved.get_value("quiz", "hide_next_track")).is_false()
 	assert_int(saved.get_value("quiz", "winning_score")).is_equal(19)
+
+
+func test_short_songs_shows_its_seconds_and_is_saved() -> void:
+	Settings.short_songs = false
+	await _open()
+	assert_bool(_card.short_song_seconds_row.visible).is_false()
+
+	_card.short_songs_button.button_pressed = true
+
+	assert_bool(Settings.short_songs).is_true()
+	assert_bool(_card.short_song_seconds_row.visible).is_true()
+	var saved: ConfigFile = ConfigFile.new()
+	assert_int(saved.load(Settings.saved_settings_path())).is_equal(OK)
+	assert_bool(saved.get_value("quiz", "short_songs")).is_true()
+
+
+func test_seconds_per_song_go_in_fives_between_ten_and_two_minutes() -> void:
+	Settings.short_song_seconds = 30
+	Settings.short_songs = true
+	await _open()
+
+	_card.increase_short_song_seconds_button.pressed.emit()
+	assert_int(Settings.short_song_seconds).is_equal(35)
+	assert_str(_card.short_song_seconds_label.text).is_equal("35")
+	for i: int in 10:
+		_card.lower_short_song_seconds_button.pressed.emit()
+	assert_int(Settings.short_song_seconds).is_equal(QuizSettingsCard.MIN_SHORT_SONG_SECONDS)
+	for i: int in 40:
+		_card.increase_short_song_seconds_button.pressed.emit()
+	assert_int(Settings.short_song_seconds).is_equal(QuizSettingsCard.MAX_SHORT_SONG_SECONDS)