fix(single): fill slide and video cover media like the image cover

Slide-mode images rendered at intrinsic ratio, top-aligned inside the
swiper chain (--pc-height:auto left .swiper-wrapper content-sized), so
the taller box showed a gap at the bottom and never center-cropped like
the image cover. The video cover had the same problem through
video-player.js (height:auto, capped at 70vh).

- aksrt_render_slider scale mode emits --pc-height/--m-height 100% so
  the wrapper/slide/img chain fills the ratio box and object-fit: cover
  center-crops inside it (same pattern as the .scale-height widgets)
- video-player.js renders the video at height:100% + object-fit:cover;
  with an undimensioned container height:100% degrades to auto,
  matching the previous behavior
- drop the dead .single-cover padding-bottom rules (every producer owns
  its height inline, and the rule squeezed the absolutely positioned
  slide inner) and the orphaned --single-cover-scale var in
  tokens.css/compat-vars.css/main.css/InlineStyleBuilder
- rebuild public bundles; extend the single-cover-height contract

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

@@ -4,14 +4,14 @@
  * ============================================================ */
 
 /* ===== Cover (in-column ratio box) — legacy .single-cover ===== */
+/* 高度由各模式的内联 padding-bottom 决定；此处不得再声明 padding-bottom，
+ * 否则会挤压绝对定位轮播内层（同样挂 .single-cover 类）的 content box。 */
 .single-cover {
     position: relative;
     overflow: hidden;
     border-radius: var(--main-radius, var(--radius, 8px));
     box-shadow: 0 0 10px var(--main-shadow, rgba(116, 116, 116, 0.08));
     margin-bottom: 20px;
-    /* legacy: padding-bottom: calc(var(--single-cover-scale) + 50px) */
-    padding-bottom: calc(var(--single-cover-scale, 35%) + 50px);
 }
 
 .single-cover > .fit-cover,