Fix Nuxt SEO OG:IMAGE not found

Use `SITE_URL` instead of `NUXT_PUBLIC_SITE_URL`, as it doesn't exist and was therefore falling back to localhost:3000 in production. `NUXT_PUBLIC_SITE_URL`  isn't noted in the README for the frontend install instructions of AgencyOS and is specific to Nuxt SEO OG:IMAGE.

nuxt.config.ts

@@ -97,7 +97,7 @@ export default defineNuxtConfig({
 	},
 
 	site: {
-		url: process.env.NUXT_PUBLIC_SITE_URL || 'http://localhost:3000',
+		url: process.env.SITE_URL || 'http://localhost:3000',
 		name: 'AgencyOS',
 	},