refactor: load env files with @next/env instead of dotenv

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

package.json

@@ -80,6 +80,7 @@
     "@content-collections/cli": "^0.1.9",
     "@content-collections/core": "^0.15.3",
     "@content-collections/next": "^0.2.11",
+    "@next/env": "16.3.8",
     "@tailwindcss/postcss": "^4.3.3",
     "@tailwindcss/typography": "^0.5.20",
     "@types/mdx": "^2.0.14",
@@ -88,7 +89,6 @@
     "@types/react": "19.3.0",
     "@types/react-dom": "19.3.0",
     "babel-plugin-react-compiler": "1.0.0",
-    "dotenv": "^18.0.5",
     "drizzle-kit": "1.0.0-rc.5-5935859",
     "oxfmt": "^0.71.0",
     "oxlint": "^1.86.0",