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

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

package.json

@@ -46,7 +46,6 @@
     "@radix-ui/react-toggle": "^1.1.10",
     "@radix-ui/react-toggle-group": "^1.1.11",
     "@radix-ui/react-tooltip": "^1.2.8",
-    "@t3-oss/env-nextjs": "^0.13.10",
     "@tanstack/react-form": "^1.28.3",
     "@vercel/analytics": "^1.6.1",
     "@vercel/functions": "^3.4.2",