Fix Vercel build error: use resolveFiles for fumadocs-mdx source

Co-authored-by: huangyiirene <7665279+huangyiirene@users.noreply.github.com>

Author: Copilot

apps/site/lib/source.ts

@@ -1,12 +1,10 @@
 import { loader } from 'fumadocs-core/source';
-import { createMDXSource } from 'fumadocs-mdx';
+import { resolveFiles } from 'fumadocs-mdx';
 import { docs, meta } from '@/.source';
 
-const mdxSource = createMDXSource(docs, meta);
-
 export const source = loader({
   baseUrl: '/docs',
   source: {
-    files: mdxSource.files(),
+    files: resolveFiles({ docs, meta }),
   },
 });