fix: add lib/utils.ts for shadcn component imports

STORY:
shadcn/ui components expect @/lib/utils to exist for the cn() helper.
Our project uses src/lib/, but shadcn creates lib/ at root.

COLLABORATION:
Created lib/utils.ts with the cn() function to satisfy shadcn imports.
This coexists with src/lib/utils.ts - both export the same function.

VERIFICATION:
- npm run typecheck: PASS

lib/utils.ts

@@ -0,0 +1,6 @@
+import { type ClassValue, clsx } from "clsx";
+import { twMerge } from "tailwind-merge";
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}