feat: Implement Calendar component, UiProvider, and a cn utility, while enhancing picker components with new icons and styles.

packages/ui/src/utils/cn.ts

@@ -0,0 +1,11 @@
+import { type ClassValue, clsx } from 'clsx';
+import { twMerge } from 'tailwind-merge';
+
+/**
+ * Intelligently merges classnames.
+ * 1. clsx: handles conditional logic (true/false)
+ * 2. twMerge: removes conflicting Tailwind classes
+ */
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}