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

src/Core/FrontSubmit.php

@@ -227,29 +227,33 @@ class FrontSubmit {
         }
         return [
             'enabled' => true,
+            'btn_text' => '发布文章',
             'title_min' => 5,
             'title_max' => 30,
+            'title_placeholder' => '请输入标题',
+            'content_placeholder' => '请输入内容',
             'allowed_cat_ids' => [],
-            'default_user_id' => 0,
-            'guest_enabled' => false,
-            'allow_category' => true,
+            'default_user_id' => 1,
             'allow_tags' => true,
-            'allow_cover' => true,
             'custom_filter_s' => true,
-            'review_mode' => 'pending',
             'pay_free_s' => true,
             'pay_price_min' => 0,
-            'pay_price_max' => 100,
+            'pay_price_max' => 0,
+            'pay_points_min' => 0,
+            'pay_points_max' => 0,
             'pay_vip_price_s' => true,
             'pay_vip_1_enabled' => false,
             'pay_vip_2_enabled' => false,
-            'pay_vip_1_discount' => 80,
-            'pay_vip_2_discount' => 50,
+            'pay_vip_1_discount' => 100,
+            'pay_vip_2_discount' => 100,
         ];
     }
 
     /**
      * Whether the user (or guest) may open / create a submission.
+     *
+     * Guest submission is granted through the capability matrix ("all" column
+     * of new_post_add), mirroring the reference theme.
      */
     public function can_submit(?int $user_id = null): bool {
         $settings = $this->settings();
@@ -261,10 +265,6 @@ class FrontSubmit {
             $user_id = get_current_user_id();
         }
 
-        if ($user_id <= 0) {
-            return !empty($settings['guest_enabled']) && (int) ($settings['default_user_id'] ?? 0) > 0;
-        }
-
         return $this->user_can_cap('new_post_add', $user_id);
     }