Make the buttons consistent and rework the player list

Buttons get 12 px of side padding. The top bar buttons, the quiz toolbar and
Show answer/Next are laid out in rows with 12 px gaps and are all 36 px tall,
and the ±1 buttons use a compact StepButton style. "Randomize new track" is now
"Next song".

Player rows show the name and the score apart, alternate like the other lists
and highlight the character under the mouse. The name field is a LineEdit with
a 10-letter limit; blank and duplicate names are refused, and a full list says
"Up to 7 players". Remove buttons update when editing is switched instead of
every frame, and the spacer panels are gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

popups/SettingsPopup.tscn

@@ -171,6 +171,7 @@ custom_minimum_size = Vector2(30, 0)
 layout_mode = 2
 focus_mode = 0
 action_mode = 0
+theme_type_variation = &"StepButton"
 text = "-1"
 
 [node name="IncreaseCacheButton" type="Button" parent="SettingsPanel/VBoxContainer/CacheRow" unique_id=1806295533]
@@ -179,6 +180,7 @@ custom_minimum_size = Vector2(30, 0)
 layout_mode = 2
 focus_mode = 0
 action_mode = 0
+theme_type_variation = &"StepButton"
 text = "+1"
 
 [node name="LowPlayedCheckButton" type="CheckButton" parent="SettingsPanel/VBoxContainer"]