fix(studio): export vercel.ts config as named 'config' not default

Per the @vercel/config docs, vercel.ts must expose a named `config`
export for Vercel's build pipeline to pick it up. The default export
was compiling to a shape Vercel rejected with a misleading
"rewrites[0] missing required property source" error.

apps/studio/vercel.ts

@@ -6,8 +6,7 @@ import type { VercelConfig } from '@vercel/config/v1'
 // these rules collapse to identity rewrites plus the shell fallback.
 const basePath = process.env.NEXT_PUBLIC_BASE_PATH ?? ''
 
-// eslint-disable-next-line no-restricted-exports
-export default {
+export const config: VercelConfig = {
   framework: null,
   outputDirectory: 'dist/client',
   rewrites: [
@@ -30,4 +29,4 @@ export default {
     // gets served the prerendered shell, which boots the client router.
     { source: `${basePath}/(.*)`, destination: '/_shell.html' },
   ],
-} satisfies VercelConfig
+}