refactor: remove @t3-oss/env-nextjs, use process.env directly

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components/layout/footer.tsx

@@ -1,6 +1,5 @@
 import Link from "next/link";
 import siteConfig from "@/lib/config/site";
-import { env } from "@/lib/env";
 
 const Footer = () => (
   <footer className="mt-8 w-full py-6 text-center text-[13px] text-muted-foreground leading-loose">
@@ -10,7 +9,7 @@ const Footer = () => (
     </Link>
     . View source on{" "}
     <a
-      href={`https://github.com/${env.NEXT_PUBLIC_GITHUB_REPO}`}
+      href={`https://github.com/${process.env.NEXT_PUBLIC_GITHUB_REPO}`}
       target="_blank"
       rel="noopener noreferrer"
       className="underline underline-offset-4"