Add assetPrefix for Tailscale funnel path routing

Assets need to be prefixed with /quietthanks so browsers request
them at the correct path when the app is served under /quietthanks.
Tailscale strips the prefix when proxying to the backend.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

next.config.ts

@@ -2,6 +2,7 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  assetPrefix: "/quietthanks",
 };
 
 export default nextConfig;