fix: add vercel.json to force Next.js framework detection

Vercel may not be detecting the Next.js framework correctly,
causing 404 on all routes. Explicit configuration should fix this.

https://claude.ai/code/session_01H2aRGDaKgarPvhay2HxN6Y

vercel.json

@@ -0,0 +1,5 @@
+{
+  "framework": "nextjs",
+  "buildCommand": "next build",
+  "installCommand": "npm install"
+}