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/EmptyState.tsx

@@ -0,0 +1,36 @@
+import * as React from 'react';
+import { cn } from '@/lib/utils/cn';
+import { LucideIcon } from 'lucide-react';
+
+export interface EmptyStateProps {
+  icon?: LucideIcon;
+  title: string;
+  description?: string;
+  action?: React.ReactNode;
+  className?: string;
+}
+
+export function EmptyState({
+  icon: Icon,
+  title,
+  description,
+  action,
+  className,
+}: EmptyStateProps) {
+  return (
+    <div className={cn('flex flex-col items-center justify-center py-12 px-4 text-center', className)}>
+      {Icon && (
+        <div className="mb-4 rounded-full bg-muted p-3">
+          <Icon className="h-6 w-6 text-muted-foreground" />
+        </div>
+      )}
+      <h3 className="mb-2 text-sm font-semibold">{title}</h3>
+      {description && (
+        <p className="mb-4 text-sm text-muted-foreground max-w-sm">
+          {description}
+        </p>
+      )}
+      {action}
+    </div>
+  );
+}