feat: launcher-style search UI (Raycast/Spotlight)

- Replace Pagefind UI with cmdk + Pagefind low-level API
- Quick actions when empty: nav (home, blog, tags) + recent posts
- Debounced full-text search with keyboard navigation
- Pass recent posts from layout to SearchModal
- Extract cn utility to lib/utils.ts
- Remove Pagefind UI styles, add Radix overlay styling
- Align blog search bar styling with launcher

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/utils.ts

@@ -0,0 +1,6 @@
+import { clsx, type ClassValue } from 'clsx';
+import { twMerge } from 'tailwind-merge';
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}