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/Button.tsx

@@ -0,0 +1,45 @@
+import * as React from 'react';
+import { cn } from '@/lib/utils/cn';
+
+export interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
+  variant?: 'default' | 'outline' | 'ghost' | 'destructive' | 'secondary';
+  size?: 'default' | 'sm' | 'lg' | 'icon';
+}
+
+const Button = React.forwardRef<HTMLButtonElement, ButtonProps>(
+  ({ className, variant = 'default', size = 'default', ...props }, ref) => {
+    return (
+      <button
+        className={cn(
+          'inline-flex items-center justify-center rounded-xl font-semibold',
+          'transition-all duration-300',
+          'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2 ring-offset-background',
+          'disabled:pointer-events-none disabled:opacity-50',
+          'active:scale-[0.98]',
+          {
+            'bg-primary text-primary-foreground shadow-[0_0_20px_rgba(139,92,246,0.3)] hover:bg-primary/90 hover:shadow-[0_0_25px_rgba(139,92,246,0.5)] hover:-translate-y-0.5':
+              variant === 'default',
+            'glass bg-white/5 border-white/10 hover:bg-white/10 hover:border-white/20 text-foreground':
+              variant === 'outline',
+            'hover:bg-accent hover:text-accent-foreground': variant === 'ghost',
+            'bg-destructive text-destructive-foreground hover:bg-destructive/90 shadow-lg hover:shadow-destructive/20':
+              variant === 'destructive',
+            'bg-secondary text-secondary-foreground hover:bg-secondary/80':
+              variant === 'secondary',
+            'h-10 px-5 py-2 text-sm': size === 'default',
+            'h-9 px-4 text-xs': size === 'sm',
+            'h-12 px-8 text-base': size === 'lg',
+            'h-10 w-10': size === 'icon',
+          },
+          className
+        )}
+        ref={ref}
+        {...props}
+      />
+    );
+  }
+);
+
+Button.displayName = 'Button';
+
+export { Button };