feat: implement Figlet, Pastel, and Unit tools with a unified layout

- Add Figlet text converter with font selection and history
- Add Pastel color palette generator and manipulation suite
- Add comprehensive Units converter with category-based logic
- Introduce AppShell with Sidebar and Header for navigation
- Modernize theme system with CSS variables and new animations
- Update project configuration and dependencies

components/ui/Skeleton.tsx

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