fix(theme): drop redundant const preloads shadowing global class_names

palette.gd / theme_keys.gd already declare `class_name`, so `const Palette =
preload(...)` in every consumer shadowed the global class and emitted 4
editor errors ("has the same name as a global class"). Reference the global
class_names directly. Editor reload now clean; suite 125/125; runtime probe
still builds all 75 showcase nodes with the theme applied.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/tests/unit/test_palette.gd

@@ -1,7 +1,5 @@
 extends "res://addons/gut/test.gd"
 
-const Palette = preload("res://scripts/theme/palette.gd")
-
 
 func test_core_tokens_are_colors():
 	assert_true(Palette.BLOOD is Color)