fix(single): align slide and video cover heights with image cover

The image-mode cover reserves +50px for the title/breadcrumb overlay
(calc(scale% + 50px)), but the slide cover wrapper was given a bare
inline padding-bottom by aksrt_render_slider, permanently defeating the
.cover-slide rule, and the video cover used a bare video_scale%.

- aksrt_render_slider: add scale_extra so scale mode emits
  calc(ratio% + extra); the single cover passes 50px so all three
  modes share one height formula
- drop the unreachable static slide fallback and function_exists guard
  (the helper is unconditionally loaded by inc/bootstrap.php), the
  always-true cover_url/ep_count conditions, and the dead --scale-ratio
  and inner swiper padding emitted in scale mode
- remove the dead cover CSS cluster with no producer: .single-cover-slide
  (main.css/main.min.css/cover.css), .single-cover-slider family
  (components/base.css), .single-cover-slide-wrap and .single-cover__*
  helpers (cover.css), and initSlideCovers in misc.js
- rebuild public bundles via tools/build-public-assets.mjs
- add single-cover-height-contract test covering the shared formula
  and the dead-code removals

inc/helpers/aksrt-slider.php

@@ -27,6 +27,7 @@ defined('ABSPATH') || exit;
  *     @type string $m_height        移动端高度 CSS 值 (默认 auto)
  *     @type bool   $scale_height    是否使用比例高度 (默认 false)
  *     @type float  $scale_ratio     比例值 (默认 0.45 = 45%)
+ *     @type string $scale_extra     比例高度附加量，如 '50px'（用于封面标题预留，默认空）
  *     @type string $wrapper_class   额外 wrapper 类
  *     @type string $slider_class    额外 slider 类
  *     @type bool   $show_nav        是否显示导航箭头 (默认 true)
@@ -61,6 +62,7 @@ function aksrt_render_slider(array $args = []): string {
         'm_height'        => 'auto',
         'scale_height'    => false,
         'scale_ratio'     => 0.45,
+        'scale_extra'     => '',
         'wrapper_class'   => '',
         'slider_class'    => '',
         'show_nav'        => true,
@@ -91,14 +93,16 @@ function aksrt_render_slider(array $args = []): string {
 
     // 高度样式
     $height_style = '';
+    $scale_padding = '';
     if ($args['scale_height']) {
         // padding-bottom percentage for ratio box (legacy graphic cover style).
+        // 内层 .new-swiper 为绝对定位铺满 wrapper，高度完全由 wrapper 的 padding-bottom 决定。
         $ratio_pct = max(10, min(100, (float) $args['scale_ratio'] * 100));
-        $height_style = sprintf(
-            '--pc-height:auto;--m-height:auto;--scale-ratio:%s;padding-bottom:%s%%;',
-            esc_attr((string) $args['scale_ratio']),
-            esc_attr((string) $ratio_pct)
-        );
+        $scale_padding = $ratio_pct . '%';
+        $scale_extra = trim((string) $args['scale_extra']);
+        if ($scale_extra !== '') {
+            $scale_padding = 'calc(' . $scale_padding . ' + ' . $scale_extra . ')';
+        }
     } else {
         $height_style = sprintf('--pc-height:%s;--m-height:%s;', esc_attr($args['pc_height']), esc_attr($args['m_height']));
     }
@@ -115,8 +119,8 @@ function aksrt_render_slider(array $args = []): string {
     $wrapper_class_str = implode(' ', $wrapper_classes);
 
     // 渲染 HTML
-    $html = '<div class="' . esc_attr($wrapper_class_str) . '"' . ($args['scale_height'] ? ' style="position:relative;' . esc_attr(sprintf('padding-bottom:%s%%;', (string) max(10, min(100, (float) $args['scale_ratio'] * 100)))) . '"' : '') . '>';
-    $html .= '<div class="' . esc_attr($slider_class_str) . '" ' . implode(' ', $data_attrs) . ' style="' . esc_attr($height_style) . ($args['scale_height'] ? 'position:absolute;inset:0;width:100%;height:100%;' : '') . '">';
+    $html = '<div class="' . esc_attr($wrapper_class_str) . '"' . ($args['scale_height'] ? ' style="position:relative;padding-bottom:' . esc_attr($scale_padding) . ';"' : '') . '>';
+    $html .= '<div class="' . esc_attr($slider_class_str) . '" ' . implode(' ', $data_attrs) . ($args['scale_height'] ? ' style="position:absolute;inset:0;width:100%;height:100%;"' : ' style="' . esc_attr($height_style) . '"') . '>';
     $html .= '<div class="swiper-wrapper">';
 
     foreach ($slides as $slide) {