Update app branding to the new shield "RZ" logo

Replaces the OS/window icon with the new dark-background logo design,
and derives a transparent shield-mark cutout (via chroma-key against
the flat background) for the in-app login/sidebar mark and the web
loading-screen logo. Brand.ranaze_mark_texture() no longer needs the
AtlasTexture crop since the new source asset is already tightly
cropped to the mark.

scripts/ui/medmind_style.gd

@@ -22,11 +22,8 @@ const LIGHT_MUTED := Color("64748b")
 static var font: Font = preload("res://assets/fonts/Inter-Variable.ttf")
 
 
-static func ranaze_mark_texture() -> AtlasTexture:
-	var texture := AtlasTexture.new()
-	texture.atlas = preload("res://assets/branding/ranaze-mark.png")
-	texture.region = Rect2(400, 150, 736, 736)
-	return texture
+static func ranaze_mark_texture() -> Texture2D:
+	return preload("res://assets/branding/ranaze-mark.png")
 
 
 static func text_color(dark: bool) -> Color: