Fix: Remove custom buildCommand, let Vercel auto-detect Next.js

FIX: Remove catch-all rewrite that routed landing page to Python API, remove dead @fontsource deps

Fix: Update Vercel configuration to use native rewrites instead of deprecated routes to resolve 404 Not Found on API endpoints

Fix: Resolve Cross-Origin Resource Sharing (CORS) pre-flight blocks for Vercel deployments via vercel.json and FastAPI router fixes