refactor: remove gh/GitHub references from tokens, classes, and component names

Rename all --color-gh-* CSS tokens to --color-*, update Tailwind class names
(bg-gh-*, text-gh-*, border-gh-*, etc.) across all TSX/TS files, rename
GhButton component and file to Button, and remove trivial descriptive comments.

src/components/statistics/StatCard.tsx

@@ -6,12 +6,12 @@ interface StatCardProps {
 
 export function StatCard({ title, value, icon }: StatCardProps) {
 	return (
-		<div className="rounded-lg border border-gh-border bg-gh-canvas p-4">
+		<div className="rounded-lg border border-border bg-canvas p-4">
 			<div className="mb-1 flex items-center gap-2">
 				<span className="text-base">{icon}</span>
-				<span className="text-xs text-gh-muted">{title}</span>
+				<span className="text-xs text-muted">{title}</span>
 			</div>
-			<div className="text-2xl font-bold text-gh-primary">{value}</div>
+			<div className="text-2xl font-bold text-primary">{value}</div>
 		</div>
 	);
 }