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/enhancements/components/base.css

@@ -1,108 +1,3 @@
-/* ============================================================
- * 7. 文章封面轮播
- * ============================================================ */
-.single-cover-slider {
-    position: relative;
-    width: 100%;
-    overflow: hidden;
-    border-radius: var(--radius-lg, 8px);
-}
-
-.single-cover-slider .swiper-slide {
-    position: relative;
-    overflow: hidden;
-}
-
-.single-cover-slider__img {
-    width: 100%;
-    height: 400px;
-    object-fit: cover;
-    display: block;
-}
-
-.single-cover-slider__overlay {
-    position: absolute;
-    inset: 0;
-    background: linear-gradient(180deg, rgba(0,0,0,0) 40%, rgba(0,0,0,0.6) 100%);
-    pointer-events: none;
-}
-
-.single-cover-slider__content {
-    position: absolute;
-    bottom: 0;
-    left: 0;
-    right: 0;
-    padding: 24px;
-    color: #fff;
-    z-index: 2;
-}
-
-.single-cover-slider__title {
-    margin: 0 0 8px;
-    font-size: 24px;
-    font-weight: 700;
-    text-shadow: 0 1px 3px rgba(0,0,0,0.3);
-}
-
-.single-cover-slider__meta {
-    display: flex;
-    gap: 16px;
-    font-size: 13px;
-    opacity: 0.9;
-}
-
-.single-cover-slider__pagination {
-    position: absolute;
-    bottom: 12px;
-    right: 16px;
-    z-index: 3;
-}
-
-.single-cover-slider__pagination .swiper-pagination-bullet {
-    background: rgba(255,255,255,0.6);
-    opacity: 1;
-}
-
-.single-cover-slider__pagination .swiper-pagination-bullet-active {
-    background: #fff;
-}
-
-.single-cover-slider__prev,
-.single-cover-slider__next {
-    position: absolute;
-    top: 50%;
-    transform: translateY(-50%);
-    z-index: 3;
-    width: 40px;
-    height: 40px;
-    border: none;
-    border-radius: 50%;
-    background: rgba(0,0,0,0.3);
-    color: #fff;
-    cursor: pointer;
-    display: flex;
-    align-items: center;
-    justify-content: center;
-    transition: background 0.2s;
-}
-
-.single-cover-slider__prev:hover,
-.single-cover-slider__next:hover {
-    background: rgba(0,0,0,0.5);
-}
-
-.single-cover-slider__prev { left: 12px; }
-.single-cover-slider__next { right: 12px; }
-
-@media (max-width: 640px) {
-    .single-cover-slider__img {
-        height: 240px;
-    }
-    .single-cover-slider__title {
-        font-size: 18px;
-    }
-}
-
 /* ============================================================
  * 8. 打赏按钮
  * ============================================================ */