fix(admin): replicate reference front-submit settings page 1:1

- Rewrite post_list/submit schema (PHP + JS) with the reference theme's
  field set, order, defaults and dependencies; topics fields excluded
- Map PostDisplay::get_submit_settings() and the FrontSubmit runtime to
  the new keys without legacy fallback
- Guest submission follows the capability matrix; category required on
  submit; pay limits read from the cash/points fieldset
- Drop dead front_submit_* settings, template branches and JS config
- Add submit settings reference contract test

assets/js/front-submit.js

@@ -131,9 +131,6 @@
 
     bindLeaveWarning: function () {
       var self = this;
-      if (cfg.leaveWarning === false) {
-        return;
-      }
       window.addEventListener('beforeunload', function (e) {
         if (self.isDirty && !self.isSubmitting && !window.post_is_save) {
           e.preventDefault();