feat: add reusable UI components for enhanced consistency

Introduce reusable UI components including `Button`, `Card`, `Accordion`, `DropdownMenu`, `HoverCard`, `Drawer`, `Avatar`, and others. Standardize styling and functionality across features to improve code maintainability and user experience.

frontend/src/components/ui/skeleton.tsx

@@ -0,0 +1,13 @@
+import { cn } from "@/lib/utils"
+
+function Skeleton({ className, ...props }: React.ComponentProps<"div">) {
+  return (
+    <div
+      data-slot="skeleton"
+      className={cn("bg-accent animate-pulse rounded-md", className)}
+      {...props}
+    />
+  )
+}
+
+export { Skeleton }