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

templates/parts/submit/sidebar-submit.php

@@ -2,12 +2,10 @@
 /** @var array $ctx */
 defined('ABSPATH') || exit;
 
-$settings   = $ctx['settings'];
 $is_can_new = !empty($ctx['is_can_new']);
 $status     = (string) $ctx['post_status'];
 $can_draft  = !empty($ctx['user_id'])
 	&& empty($ctx['is_guest'])
-	&& !empty($settings['draft_save'])
 	&& !in_array($status, ['publish', 'pending'], true);
 ?>
 <div class="zib-widget">