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>

popups/SettingsPopup.tscn

@@ -205,12 +205,31 @@ action_mode = 0
 theme_type_variation = &"StepButton"
 text = "+1"
 
-[node name="LowPlayedCheckButton" type="CheckButton" parent="SettingsPanel/VBoxContainer"]
-unique_name_in_owner = true
+[node name="RandomModeRow" type="HBoxContainer" parent="SettingsPanel/VBoxContainer"]
 layout_mode = 2
+
+[node name="Title" type="Label" parent="SettingsPanel/VBoxContainer/RandomModeRow"]
+layout_mode = 2
+size_flags_horizontal = 3
+theme_type_variation = &"SettingsRowLabel"
+text = "Random songs"
+
+[node name="RandomModeButton" type="OptionButton" parent="SettingsPanel/VBoxContainer/RandomModeRow"]
+unique_name_in_owner = true
+custom_minimum_size = Vector2(180, 0)
+layout_mode = 2
+tooltip_text = "Normal: a random soundtrack, then one of its songs.
+Low played: the same, but soundtracks played less than average come up more often.
+Classic: every song has the same chance, whatever its soundtrack."
 focus_mode = 0
-action_mode = 0
-text = "Prefer less played soundtracks"
+selected = 0
+item_count = 3
+popup/item_0/text = "Normal"
+popup/item_0/id = 0
+popup/item_1/text = "Low played"
+popup/item_1/id = 1
+popup/item_2/text = "Classic"
+popup/item_2/id = 2
 
 [node name="AddToDatabaseCheckButton" type="CheckButton" parent="SettingsPanel/VBoxContainer"]
 unique_name_in_owner = true