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

.gitignore

@@ -50,6 +50,7 @@
 !/tests/widget-system/comment-image-upload-contract.test.cjs
 !/tests/widget-system/pay-card-contract.test.cjs
 !/tests/widget-system/circular-button-icon-margin-contract.test.cjs
+!/tests/widget-system/single-cover-height-contract.test.cjs
 !/tests/widget-system/frontend-lifecycle-contract.test.cjs
 !/tests/asset-ledger/
 !/tests/asset-ledger/*.php