feat(forum): guest cash purchase for topic and plate follow

- Reuse pay_no_logged_in / pay_cookie_day; generalized guest cookie helpers
- Topic + plate cash: guest preview/purchase (no balance), order.user_id=0, cookie unlock
- View gates: can_view_topic pay + plate follow accept signed guest cookies
- Templates: guest buy/follow CTAs; poll endpoint allows plate_follow guest orders

inc/classes/forum/board-service-traits/trait-aksrt-forum-board-service-part-2.php

@@ -209,6 +209,13 @@ trait AKSRT_Forum_Board_Service_Part_2 {
         }
 
         if ($user_id <= 0) {
+            // Guest cash follow unlock via signed cookie (money follow pay only).
+            if ($allow_view === 'follow'
+                && function_exists('aksrt_guest_has_plate_follow_access')
+                && aksrt_guest_has_plate_follow_access($plate_id)
+            ) {
+                return true;
+            }
             return false;
         }
 
@@ -282,9 +289,13 @@ trait AKSRT_Forum_Board_Service_Part_2 {
      * Check if a user is following a plate (§7.2 关注).
      */
     public function is_user_following_plate(int $user_id, int $plate_id): bool {
-        if ($user_id <= 0) {
+        if ($plate_id <= 0) {
             return false;
         }
+        if ($user_id <= 0) {
+            return function_exists('aksrt_guest_has_plate_follow_access')
+                && aksrt_guest_has_plate_follow_access($plate_id);
+        }
         $follow_list = get_user_meta($user_id, 'aksrt_follow_plate', true);
         if (!is_array($follow_list)) {
             return false;