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/js/core/misc.js

@@ -99,50 +99,6 @@
 
 })();
 
-(function () {
-  "use strict";
-
-  function initSlideCovers() {
-    var covers = document.querySelectorAll(".single-cover--slide");
-    covers.forEach(function (cover) {
-      var slides = cover.querySelectorAll(".single-cover__slide");
-      var dots = cover.querySelectorAll(".single-cover__dot");
-      if (slides.length < 2) return;
-
-      var current = 0;
-      var interval = 5000;
-
-      function goTo(idx) {
-        slides[current].classList.remove("is-active");
-        if (dots[current]) dots[current].classList.remove("is-active");
-        current = idx;
-        slides[current].classList.add("is-active");
-        if (dots[current]) dots[current].classList.add("is-active");
-      }
-
-      var timer = setInterval(function () {
-        goTo((current + 1) % slides.length);
-      }, interval);
-
-      dots.forEach(function (dot, i) {
-        dot.addEventListener("click", function () {
-          clearInterval(timer);
-          goTo(i);
-          timer = setInterval(function () {
-            goTo((current + 1) % slides.length);
-          }, interval);
-        });
-      });
-    });
-  }
-
-  if (document.readyState === "loading") {
-    document.addEventListener("DOMContentLoaded", initSlideCovers);
-  } else {
-    initSlideCovers();
-  }
-})();
-
 (function () {
   "use strict";