WIP checkpoint: fix script-attach typo, missing class_name, var name mismatch

- scenes/avatar_customization.tscn: script_extense -> script (typo meant
  the customization UI script was never actually attached to the scene)
- scripts/avatar/avatar_loadout.gd: add missing class_name AvatarLoadout
  (avatar_system.gd references the type everywhere; only this file among
  the avatar scripts lacked its class_name)
- scripts/main_menu.gd: _ready() referenced _customize_button/_settings_button/
  _quit_button but the @onready vars are named without the leading
  underscore

scenes/avatar_customization.tscn

@@ -48,7 +48,7 @@ anchor_right = 1.0
 anchor_bottom = 1.0
 grow_horizontal = 2
 grow_vertical = 2
-script_extense = ExtResource("1")
+script = ExtResource("1")
 
 [node name="MainPanel" type="Panel" parent="."]
 layout_mode = 3