chore: Improve Helpcenter custom domains (#5456)

- Support rendering articles over frontend URL
- Support rendering articles over help center URL
- Support rendering help center home page in the custom domain root

app/controllers/public_controller.rb

@@ -7,14 +7,15 @@ class PublicController < ActionController::Base
   private
 
   def ensure_custom_domain_request
-    custom_domain = request.host
+    domain = request.host
 
-    @portals = ::Portal.where(custom_domain: custom_domain)
+    return if [URI.parse(ENV.fetch('FRONTEND_URL', '')).host, URI.parse(ENV.fetch('HELPCENTER_URL', '')).host].include?(domain)
 
-    return if @portals.present?
+    @portal = ::Portal.find_by(custom_domain: domain)
+    return if @portal.present?
 
     render json: {
-      error: "Domain: #{custom_domain} is not registered with us. \
+      error: "Domain: #{domain} is not registered with us. \
       Please send us an email at support@chatwoot.com with the custom domain name and account API key"
     }, status: :unauthorized and return
   end