fix: rename middleware.mjs to middleware.js, revert framework override

Try .js extension for middleware detection. Revert framework: null
since the Docusaurus preset shouldn't block middleware.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: lennessyy

middleware.mjs middleware.jsmiddleware.mjs renamed to middleware.js

@@ -1,9 +1,6 @@
 {
   "public": true,
   "trailingSlash": false,
-  "framework": null,
-  "buildCommand": "docusaurus build",
-  "outputDirectory": "build",
   "github": {
     "silent": true
   },