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/unit/test_settings_files.gd

@@ -8,6 +8,7 @@ const SAVED_VALUES: Dictionary = {
 	&"hide_ticks": true, &"winning_score": 13, &"inspiration_list_speed": 4,
 	&"fullscreen": true, &"last_seen_version": "1.2.3", &"theme_id": "original_web",
 	&"reduce_animations": true,
+	&"short_songs": true, &"short_song_seconds": 45,
 }
 
 var _snapshot: AutoloadSnapshot
@@ -93,7 +94,7 @@ func test_statistics_settings_are_not_saved() -> void:
 
 	var file: ConfigFile = _saved_file()
 	assert_bool(file.has_section_key("quiz", "add_to_stats")).is_false()
-	assert_bool(file.has_section_key("quiz", "use_low_played_mode")).is_false()
+	assert_bool(file.has_section_key("quiz", "random_mode")).is_false()
 
 
 func test_queue_ids_are_saved_and_load_back() -> void: