refactor(e5): deferred review-tail (frontend) — Carousel reuse + shared scroll core

The last of the deep-review deferred reuse items:

- extract the scroll-affordance machinery (watch the scroller AND its swappable/
  lazily-growing children, diff two edge booleans) that useScrollFade (vertical) and
  Carousel's useHScroll (horizontal) each hand-duplicated into one shared
  lib/useScrollEdges(axis); both hooks now just map start/end to their own shape.

- migrate PlexInfo's two remaining hand-rolled `flex overflow-x-auto` strips — the
  cast row and the collection strips — to the S1 <Carousel> primitive, so they get the
  edge fade + hover/focus scroll buttons (only on the clipped side) for free, matching
  the Discover rails / CastStrip / RelatedStrip.

Pure refactor, no behaviour change. Gate green (tsc, vitest 351, eslint, prettier,
knip). Browser-verified on a movie info page: cast row + collection strip render as
Carousels with the edge fade and the right-scroll button on the clipped cast row.

frontend/src/lib/useScrollFade.ts

@@ -1,15 +1,15 @@
-import { useEffect, useState, type CSSProperties } from "react";
+import { type CSSProperties } from "react";
+import { useScrollEdges } from "./useScrollEdges";
 
-// Scroll-affordance for a scroll container (typically one whose native scrollbar is hidden with
-// `.no-scrollbar`, but the page scroller keeps its bar and takes the fade anyway):
-// fades the top/bottom edge of the content to hint there's more to scroll, but only on the edge
-// that's actually clipped (no fade at a boundary, none when it all fits). Returns a callback ref
-// to attach to the scroller and the mask `style` to spread onto it. Uses a node-state callback
-// ref (not useRef) so the effect (re)runs whenever the element attaches — e.g. a SidePanel that
-// mounts collapsed and only renders its scroll body once expanded still wires up correctly.
-// `fadeTop` gates the top edge only. Pass false when the scroller's top is a sticky element (a
-// table's sticky header): fading there would eat into the header instead of hinting at content, and
-// the header would visibly change as the fade turns on with the first scroll.
+// Vertical scroll-affordance for a scroll container (typically one whose native scrollbar is hidden
+// with `.no-scrollbar`, but the page scroller keeps its bar and takes the fade anyway): fades the
+// top/bottom edge of the content to hint there's more to scroll, but only on the edge that's actually
+// clipped (no fade at a boundary, none when it all fits). Returns a callback ref to attach to the
+// scroller and the mask `style` to spread onto it. The scroller-and-children watching machinery lives
+// in the shared `useScrollEdges` (the horizontal twin, Carousel's `useHScroll`, uses the same core).
+// `fadeTop` gates the top edge only. Pass false when the scroller's top is a sticky element (a table's
+// sticky header): fading there would eat into the header instead of hinting at content, and the header
+// would visibly change as the fade turns on with the first scroll.
 export function useScrollFade(
   fadePx = 20,
   fadeTop = true,
@@ -17,52 +17,9 @@ export function useScrollFade(
   ref: (node: HTMLElement | null) => void;
   style: CSSProperties;
 } {
-  const [el, setEl] = useState<HTMLElement | null>(null);
-  const [fade, setFade] = useState({ up: false, down: false });
-
-  useEffect(() => {
-    if (!el) return;
-    // Bail out when neither edge changed: `scroll` fires per frame, and the page scroller's
-    // hook sits above the whole page tree, so a new object here would re-render on every frame
-    // of every scroll. The two booleans only flip at the ends of the range.
-    const update = () => {
-      const up = el.scrollTop > 1;
-      const down = el.scrollTop + el.clientHeight < el.scrollHeight - 1;
-      setFade((prev) => (prev.up === up && prev.down === down ? prev : { up, down }));
-    };
-    update();
-    el.addEventListener("scroll", update, { passive: true });
-    // Watch the scroller AND its content. Content that grows or shrinks — a lazy page loading in,
-    // a thread's messages decrypting, a list being filtered — changes neither the scroller's own
-    // box nor its scrollTop, so without the children nothing would re-evaluate the edges.
-    const ro = new ResizeObserver(update);
-    ro.observe(el);
-    for (const child of Array.from(el.children)) ro.observe(child);
-    // Children get SWAPPED, and a ResizeObserver holding a detached node simply never fires
-    // again. Measured: the Plex page opened with no bottom fade over a 3119px list, because the
-    // node observed at mount was the Suspense fallback the real page then replaced. (Same
-    // lazy-page race that broke BackToTop.) So follow the child list: the records name exactly
-    // which nodes arrived and left, which keeps this O(1) per change rather than re-observing
-    // every child of a long list on each append.
-    const mo = new MutationObserver((records) => {
-      for (const rec of records) {
-        for (const node of Array.from(rec.addedNodes))
-          if (node instanceof Element) ro.observe(node);
-        for (const node of Array.from(rec.removedNodes))
-          if (node instanceof Element) ro.unobserve(node);
-      }
-      update();
-    });
-    mo.observe(el, { childList: true });
-    return () => {
-      el.removeEventListener("scroll", update);
-      ro.disconnect();
-      mo.disconnect();
-    };
-  }, [el]);
-
-  const mask = `linear-gradient(to bottom, transparent, #000 ${fade.up && fadeTop ? fadePx : 0}px, #000 calc(100% - ${
-    fade.down ? fadePx : 0
+  const { ref, start, end } = useScrollEdges("y");
+  const mask = `linear-gradient(to bottom, transparent, #000 ${start && fadeTop ? fadePx : 0}px, #000 calc(100% - ${
+    end ? fadePx : 0
   }px), transparent)`;
-  return { ref: setEl, style: { maskImage: mask, WebkitMaskImage: mask } };
+  return { ref, style: { maskImage: mask, WebkitMaskImage: mask } };
 }