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/components/Carousel.tsx

@@ -1,7 +1,8 @@
-import { useCallback, useEffect, useState, type ReactNode } from "react";
+import { useCallback, type ReactNode } from "react";
 import { ChevronLeft, ChevronRight } from "lucide-react";
 import { useTranslation } from "react-i18next";
 import clsx from "clsx";
+import { useScrollEdges } from "../lib/useScrollEdges";
 
 // Shared horizontal rail — the one place the app's "row of cards you scroll sideways" lives, replacing
 // the hand-rolled `flex gap overflow-x-auto` strips (Cast / Related / collections) and the future
@@ -19,50 +20,18 @@ function scrollBehavior(): ScrollBehavior {
   return reduce ? "auto" : "smooth";
 }
 
-// Horizontal scroll-affordance: tracks whether the scroller is clipped on the left / right so the rail
-// can show a button + fade only where there's more to see. The X-axis twin of useScrollFade — same
-// node-state callback ref (re-runs on attach) and same watch-the-children discipline (a rail whose
-// cards load in later grows scrollWidth without ever firing a scroll event), and it also hands back the
-// element so the buttons can scroll it.
+// Horizontal scroll-affordance: whether the scroller is clipped on the left / right, so the rail shows
+// a button + fade only where there's more to see. The X-axis face of the shared `useScrollEdges` core
+// (the same watch-the-scroller-and-its-children discipline as the vertical `useScrollFade`), plus the
+// element itself so the paging buttons can scroll it.
 function useHScroll(): {
   ref: (node: HTMLElement | null) => void;
   el: HTMLElement | null;
   left: boolean;
   right: boolean;
 } {
-  const [el, setEl] = useState<HTMLElement | null>(null);
-  const [edge, setEdge] = useState({ left: false, right: false });
-
-  useEffect(() => {
-    if (!el) return;
-    const update = () => {
-      const left = el.scrollLeft > 1;
-      const right = el.scrollLeft + el.clientWidth < el.scrollWidth - 1;
-      setEdge((prev) => (prev.left === left && prev.right === right ? prev : { left, right }));
-    };
-    update();
-    el.addEventListener("scroll", update, { passive: true });
-    const ro = new ResizeObserver(update);
-    ro.observe(el);
-    for (const child of Array.from(el.children)) ro.observe(child);
-    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]);
-
-  return { ref: setEl, el, left: edge.left, right: edge.right };
+  const { ref, el, start, end } = useScrollEdges("x");
+  return { ref, el, left: start, right: end };
 }
 
 const FADE_PX = 28;

frontend/src/components/PlexInfo.tsx

@@ -15,6 +15,7 @@ import {
   useDetailPrefs,
 } from "../lib/plexDetailUi";
 import { usePref } from "../lib/usePref";
+import { Carousel } from "./Carousel";
 import PlexCollectionEditor from "./PlexCollectionEditor";
 import PlexPlaylistAdd from "./PlexPlaylistAdd";
 
@@ -614,7 +615,7 @@ export default function PlexInfo({
             <h2 className={`mb-2 text-sm font-semibold ${overlay ? "text-white/80" : ""}`}>
               {t("plex.info.cast")}
             </h2>
-            <div className="flex gap-3 overflow-x-auto pb-2">
+            <Carousel ariaLabel={t("plex.info.cast")} gap="gap-3">
               {cast.map((c, i) => {
                 const clickable = !!(onOpenPerson || onFilter);
                 const inner = (
@@ -669,7 +670,7 @@ export default function PlexInfo({
                   </div>
                 );
               })}
-            </div>
+            </Carousel>
           </div>
         )}
 
@@ -694,7 +695,7 @@ export default function PlexInfo({
                   </button>
                 )}
               </div>
-              <div className="flex gap-3 overflow-x-auto pb-2">
+              <Carousel ariaLabel={col.title} gap="gap-3">
                 {col.items.map((m) => (
                   <button
                     key={m.id}
@@ -739,7 +740,7 @@ export default function PlexInfo({
                     {m.year ? <div className="text-2xs text-muted">{m.year}</div> : null}
                   </button>
                 ))}
-              </div>
+              </Carousel>
             </div>
           ))}
       </div>