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

assets/css/frontend/single-post/cover.css

@@ -14,11 +14,6 @@
     padding-bottom: calc(var(--single-cover-scale, 35%) + 50px);
 }
 
-.single-cover-slide {
-    padding-bottom: calc(var(--scale-height, var(--single-cover-scale, 35%)) + 50px);
-    border-radius: var(--main-radius, var(--radius, 8px));
-}
-
 .single-cover > .fit-cover,
 .single-cover > img {
     position: absolute;
@@ -46,118 +41,15 @@
     font-weight: 700;
 }
 
-/* Legacy overlay helpers (kept for non-legacy markup) */
-.single-cover__overlay {
-    position: absolute;
-    inset: 0;
-    background: linear-gradient(
-        to bottom,
-        rgba(0, 0, 0, 0) 30%,
-        rgba(0, 0, 0, 0.6) 100%
-    );
-    pointer-events: none;
-}
-
-.single-cover__content {
-    position: absolute;
-    bottom: 0;
-    left: 0;
-    right: 0;
-    padding: 24px;
-    color: #fff;
-    border-radius: 0 0 var(--main-radius, var(--radius, 8px)) var(--main-radius, var(--radius, 8px));
-}
-
-.single-cover__title,
-.single-cover__content .article-title {
-    font-size: clamp(1.125rem, 2.5vw, 1.5rem);
-    font-weight: 700;
-    margin: 0 0 6px;
-    line-height: 1.35;
-    text-shadow: 0 1px 4px rgba(0, 0, 0, 0.4);
-    color: #fff;
-}
-
-.single-cover__meta {
-    display: flex;
-    flex-wrap: wrap;
-    gap: 12px;
-    font-size: 12px;
-    opacity: 0.85;
-}
-
-.single-cover__meta span {
-    display: inline-flex;
-    align-items: center;
-    gap: 4px;
-}
-
 /* ===== Video Cover ===== */
 .single-cover--video {
     background: #000;
 }
 
-.single-cover--video .single-cover__video {
-    position: absolute;
-    inset: 0;
-    width: 100%;
-    height: 100%;
-    object-fit: cover;
-}
-
-/* ===== Slide Cover ===== */
 .single-cover--slide {
     background: var(--color-bg-2, #f5f5f5);
 }
 
-.single-cover__slides {
-    position: absolute;
-    inset: 0;
-    width: 100%;
-    height: 100%;
-}
-
-.single-cover__slide {
-    position: absolute;
-    inset: 0;
-    opacity: 0;
-    transition: opacity 0.6s ease;
-}
-
-.single-cover__slide.is-active {
-    opacity: 1;
-}
-
-.single-cover__slide img {
-    width: 100%;
-    height: 100%;
-    object-fit: cover;
-}
-
-.single-cover__dots {
-    position: absolute;
-    bottom: 14px;
-    right: 18px;
-    display: flex;
-    gap: 8px;
-    z-index: 2;
-}
-
-.single-cover__dot {
-    width: 9px;
-    height: 9px;
-    border-radius: 50%;
-    background: rgba(255, 255, 255, 0.45);
-    border: 1px solid rgba(255, 255, 255, 0.32);
-    transition: background 0.3s ease, transform 0.3s ease;
-    cursor: pointer;
-}
-
-.single-cover__dot.is-active {
-    background: #fff;
-    transform: scale(1.12);
-}
-
 /* ===== 面包屑 ===== */
 .post-breadcrumbs {
     margin-bottom: 12px;
@@ -240,16 +132,6 @@
   margin-right: 3px;
   opacity: 0.9;
 }
-.single-cover-slide-wrap {
-  position: relative;
-  overflow: hidden;
-  border-radius: var(--main-radius, var(--radius, 8px));
-}
-.single-cover-slide-wrap .abs-center.left-bottom {
-  z-index: 2;
-  padding: 15px 15px 10px 15px;
-  color: #fff;
-}
 .article-avatar name.flex {
   display: inline-flex;
   align-items: center;