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

templates/forum/forum-topic.php

@@ -192,7 +192,12 @@ if (empty($topic['can_view'])) :
                     <?php elseif ($charge_amount > 0 || $base_amount > 0) : ?>
                         <span class="aksrt-forum-topic__pay-price"><?php printf(esc_html__('价格：¥%s', 'aksrt'), esc_html(number_format(max($charge_amount, $base_amount), 2, '.', ''))); ?></span>
                     <?php endif; ?>
-                    <?php if (!is_user_logged_in()) : ?>
+                    <?php
+                    $allow_guest_topic = !is_user_logged_in()
+                        && function_exists('aksrt_allow_guest_forum_pay')
+                        && aksrt_allow_guest_forum_pay(get_post($topic_id));
+                    ?>
+                    <?php if (!is_user_logged_in() && !$allow_guest_topic) : ?>
                         <a href="<?php echo esc_url(wp_login_url(get_permalink($topic_id))); ?>" class="aksrt-forum-topic__primary-btn"><?php esc_html_e('登录后购买', 'aksrt'); ?></a>
                     <?php else : ?>
                         <button type="button" class="aksrt-forum-topic__primary-btn"
@@ -201,13 +206,19 @@ if (empty($topic['can_view'])) :
                             data-free="<?php echo $is_free_vip ? '1' : '0'; ?>">
                             <i class="fas fa-shopping-cart"></i>
                             <?php
-                            if ($is_free_vip) {
+                            if ($is_free_vip && is_user_logged_in()) {
                                 esc_html_e('VIP 免费解锁', 'aksrt');
                             } else {
-                                printf(esc_html__('购买解锁 ¥%s', 'aksrt'), esc_html(number_format($charge_amount, 2, '.', '')));
+                                printf(esc_html__('购买解锁 ¥%s', 'aksrt'), esc_html(number_format(max($charge_amount, $base_amount), 2, '.', '')));
                             }
                             ?>
                         </button>
+                        <?php if ($allow_guest_topic) : ?>
+                            <p class="description" style="margin-top:8px;">
+                                <?php esc_html_e('未登录购买将通过浏览器 Cookie 解锁；建议登录后购买以便保存订单。', 'aksrt'); ?>
+                                <a href="<?php echo esc_url(wp_login_url(get_permalink($topic_id))); ?>"><?php esc_html_e('去登录', 'aksrt'); ?></a>
+                            </p>
+                        <?php endif; ?>
                         <span class="aksrt-forum-topic__unlock-feedback js-aksrt-action-feedback" aria-live="polite"></span>
                     <?php endif; ?>
                 <?php else : ?>