Add assistant-ui registry URL to registries.json (#8312)

Author: AVGVSTVS96

apps/v4/public/r/registries.json

@@ -4,6 +4,7 @@
   "@alexcarpenter": "https://ui.alexcarpenter.me/r/{name}.json",
   "@alpine": "https://alpine-registry.vercel.app/r/{name}.json",
   "@animate-ui": "https://animate-ui.com/r/{name}.json",
+  "@assistant-ui": "https://r.assistant-ui.com/{name}.json",
   "@blocks": "https://blocks.so/r/{name}.json",
   "@clerk": "https://clerk.com/r/{name}.json",
   "@cult-ui": "https://cult-ui.com/r/{name}.json",