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/layout/AppShell.tsx

@@ -0,0 +1,28 @@
+'use client';
+
+import * as React from 'react';
+import { AppSidebar } from './AppSidebar';
+import { AppHeader } from './AppHeader';
+import AnimatedBackground from '@/components/AnimatedBackground';
+import { SidebarProvider } from './SidebarProvider';
+
+interface AppShellProps {
+  children: React.ReactNode;
+}
+
+export function AppShell({ children }: AppShellProps) {
+  return (
+    <SidebarProvider>
+      <div className="flex min-h-screen bg-background text-foreground relative">
+        <AnimatedBackground />
+        <AppSidebar />
+        <div className="flex-1 flex flex-col min-w-0 relative z-10">
+          <AppHeader />
+          <main className="flex-1 overflow-y-auto">
+            {children}
+          </main>
+        </div>
+      </div>
+    </SidebarProvider>
+  );
+}