Bump to 0.21.0-beta.1 (#1251)

package.json

 {
   "name": "dyad",
   "productName": "dyad",
-  "version": "0.20.0",
+  "version": "0.21.0-beta.1",
   "description": "Free, local, open-source AI app builder",
   "main": ".vite/build/main.js",
   "repository": {