feat(theme): parchment/dark-bay/vignette shader surfaces as drop-in scenes

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

client/assets/theme/shaders/vignette.gdshader

@@ -0,0 +1,6 @@
+shader_type canvas_item;
+uniform float strength = 0.62;
+void fragment() {
+	float d = distance(UV, vec2(0.5));
+	COLOR = vec4(0.0, 0.0, 0.0, clamp(d * strength, 0.0, 1.0));
+}