fix: add frontend/src/lib/ to git (was ignored by /lib/ pattern)

The root .gitignore had `lib/` which matched frontend/src/lib/,
causing Docker build to fail with "Module not found: Can't resolve
'@/lib/utils'" and '@/lib/i18n'.

Changed to `/lib/` so it only ignores the Python lib at repo root.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/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));
+}