feat: add page_settings JSON property + ACE JSON mode

- Add property :page_settings (Hash, default {}) to Page model
- JSON.parse string input in setter (form submits as text)
- Add collapsible 'Page Settings (JSON)' section to page form
- Support data-mode='json' attribute for ACE editor
  (html_edit.js.coffee: check textarea data-mode attribute)

lib/cmtool/includes/page.rb

@@ -21,6 +21,13 @@ module Cmtool
         klass.property :active, type: :boolean, default: true
         klass.property :layout
         klass.property :in_menu, type: :boolean, default: true
+        klass.property :page_settings, default: {}
+
+        # Parse JSON string from form submission
+        klass.send(:define_method, :page_settings=) do |val|
+          val = JSON.parse(val) if val.is_a?(String)
+          super(val)
+        end
 
         klass.has_ancestry by_property: :locale