refactor: moves util functions into their respective library file

www/src/lib/posts.ts

@@ -2,6 +2,11 @@ import type { CollectionEntry } from 'astro:content';
 
 type Post = CollectionEntry<'md'>;
 
+export function getSlug(postId: string): string {
+  const parts = postId.split('/');
+  return parts[parts.length - 1];
+}
+
 export function sortPosts(posts: Post[]): Post[] {
   return posts.slice().sort((a, b) => {
     if (a.data.pinned && !b.data.pinned) return -1;