feat: set default domain for thirdweb auth (#5196)

## Problem solved

Addresses CNCT-2132

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## PR-Codex overview
This PR updates the `domain` configuration in the `createAuth` function for the `thirdwebAuth` object, changing its default value to "thirdweb.com".

### Detailed summary
- Updated the `domain` property in `createAuth` from an empty string to "thirdweb.com".

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Author: joaquim-verges

apps/playground-web/src/app/connect/auth/server/actions/auth.ts

@@ -8,7 +8,7 @@ import { generateAccount, privateKeyToAccount } from "thirdweb/wallets";
 const privateKey = process.env.THIRDWEB_ADMIN_PRIVATE_KEY;
 
 const thirdwebAuth = createAuth({
-  domain: process.env.NEXT_PUBLIC_THIRDWEB_AUTH_DOMAIN || "",
+  domain: process.env.NEXT_PUBLIC_THIRDWEB_AUTH_DOMAIN || "thirdweb.com",
   client: THIRDWEB_CLIENT,
   adminAccount: privateKey
     ? privateKeyToAccount({ client: THIRDWEB_CLIENT, privateKey })