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

app/(app)/layout.tsx

@@ -0,0 +1,16 @@
+import { AppShell } from '@/components/layout/AppShell';
+import { Providers } from '@/components/providers/Providers';
+
+export default function AppLayout({
+  children,
+}: Readonly<{
+  children: React.ReactNode;
+}>) {
+  return (
+    <Providers>
+      <AppShell>
+        {children}
+      </AppShell>
+    </Providers>
+  );
+}