chore(ui): display branch in footer if present

app/helpers/application_helper.rb

@@ -100,4 +100,11 @@ module ApplicationHelper
     end.html_safe
   end
 
+  def postal_version_string
+    string = Postal.version
+    string += " (#{Postal.branch})" if Postal.branch &&
+                                       Postal.branch != "main"
+    string
+  end
+
 end