Switch from standalone to standard Next.js build

Removes output: "standalone" from next.config.js, updates build/start/preview
scripts to use next build/next start, and updates Dockerfile to copy .next
output directory and run via bun run start instead of server.js.

https://claude.ai/code/session_014126WHVRT8mftmqkU6dajG

next.config.js

@@ -6,7 +6,6 @@ import "./src/env.js";
 
 /** @type {import("next").NextConfig} */
 const config = {
-  output: "standalone",
   reactCompiler: false,
   serverExternalPackages: ["pg", "better-auth"],
 };