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/pastel/layout/ThemeToggle.tsx

@@ -0,0 +1,28 @@
+'use client';
+
+import { Moon, Sun } from 'lucide-react';
+import { useTheme } from '@/components/providers/ThemeProvider';
+import { Button } from '@/components/ui/Button';
+
+export function ThemeToggle() {
+  const { theme, setTheme, resolvedTheme } = useTheme();
+
+  const toggleTheme = () => {
+    setTheme(resolvedTheme === 'dark' ? 'light' : 'dark');
+  };
+
+  return (
+    <Button
+      variant="ghost"
+      size="icon"
+      onClick={toggleTheme}
+      aria-label={`Switch to ${resolvedTheme === 'dark' ? 'light' : 'dark'} mode`}
+    >
+      {resolvedTheme === 'dark' ? (
+        <Sun className="h-5 w-5" />
+      ) : (
+        <Moon className="h-5 w-5" />
+      )}
+    </Button>
+  );
+}