feat(frontend): Complete shadcn migration of skeleton and styling components

Co-authored-by: elisiariocouto <818914+elisiariocouto@users.noreply.github.com>

frontend/src/components/ui/skeleton.tsx

@@ -0,0 +1,14 @@
+import { cn } from "@/lib/utils";
+
+interface SkeletonProps extends React.HTMLAttributes<HTMLDivElement> {}
+
+function Skeleton({ className, ...props }: SkeletonProps) {
+  return (
+    <div
+      className={cn("animate-pulse rounded-md bg-muted", className)}
+      {...props}
+    />
+  );
+}
+
+export { Skeleton };