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/QuizSettingsCard.tscn

@@ -50,6 +50,49 @@ focus_mode = 0
 action_mode = 0
 text = "Hide next track"
 
+[node name="ShortSongsCheckButton" type="CheckButton" parent="CardPanel/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+tooltip_text = "Play only a part of each song, from a random point, so more songs fit in a game"
+focus_mode = 0
+action_mode = 0
+text = "Short songs"
+
+[node name="ShortSongSecondsRow" type="HBoxContainer" parent="CardPanel/VBox"]
+unique_name_in_owner = true
+layout_mode = 2
+
+[node name="Title" type="Label" parent="CardPanel/VBox/ShortSongSecondsRow"]
+layout_mode = 2
+size_flags_horizontal = 3
+theme_type_variation = &"SettingsRowLabel"
+text = "Seconds per song"
+
+[node name="ShortSongSecondsLabel" type="Label" parent="CardPanel/VBox/ShortSongSecondsRow"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(48, 0)
+layout_mode = 2
+text = "30"
+horizontal_alignment = 1
+
+[node name="LowerShortSongSecondsButton" type="Button" parent="CardPanel/VBox/ShortSongSecondsRow"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(30, 0)
+layout_mode = 2
+focus_mode = 0
+action_mode = 0
+theme_type_variation = &"StepButton"
+text = "-5"
+
+[node name="IncreaseShortSongSecondsButton" type="Button" parent="CardPanel/VBox/ShortSongSecondsRow"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(30, 0)
+layout_mode = 2
+focus_mode = 0
+action_mode = 0
+theme_type_variation = &"StepButton"
+text = "+5"
+
 [node name="HideBeginningCheckButton" type="CheckButton" parent="CardPanel/VBox"]
 unique_name_in_owner = true
 layout_mode = 2