refactor(ui): reuse the store, media-query and deleted-tag rules the review found rebuilt, and name a loading pick

E32 review round 1 (F3–F6).

- Channels' open-row store and the chat dock use lib/store's createStore instead of a hand-rolled
  listener set (the chat dock gets a test of its own).
- The Plex backdrop reads prefers-reduced-motion through useMediaQuery; the query string is one
  constant (REDUCED_MOTION) for the hook and the carousel's one-off read.
- A link-restored collection filter showed its raw rating_key in the picker while the titles
  loaded: usePlexCollectionTitles now says it is pending, the picker draws a placeholder
  (labelsPending, as the channel picker does) and so does the chip row.
- The "deleted tag" rule was written twice with two i18n keys and had drifted: the channel manager
  showed NO chip for a pick while the tag list loaded, or ever after a failed load, while the pick
  still narrowed the table. One rule now (lib/tagPicks), one key (common.deletedTag); the manager's
  row shows every pick, as a placeholder while loading and as its id after a failure.

frontend/src/lib/plexDetail.ts

@@ -1,4 +1,5 @@
-import { useLayoutEffect, useSyncExternalStore } from "react";
+import { useLayoutEffect } from "react";
+import { REDUCED_MOTION, useMediaQuery } from "./useMediaQuery";
 import { usePref, useSavePref } from "./usePref";
 import type { TFunction } from "i18next";
 import type { PlexCard, PlexTmdb } from "./api";
@@ -7,18 +8,6 @@ import type { PlexCard, PlexTmdb } from "./api";
 // prefs, the TMDB backdrop URL, the crossfading art backdrop hook and the episode label. Split out so
 // plexDetailUi exports only components (Fast Refresh).
 
-const REDUCED_MOTION = "(prefers-reduced-motion: reduce)";
-const reducedMotion = () =>
-  typeof window !== "undefined" &&
-  typeof window.matchMedia === "function" &&
-  window.matchMedia(REDUCED_MOTION).matches;
-const onReducedMotionChange = (cb: () => void) => {
-  if (typeof window === "undefined" || typeof window.matchMedia !== "function") return () => {};
-  const m = window.matchMedia(REDUCED_MOTION);
-  m.addEventListener("change", cb);
-  return () => m.removeEventListener("change", cb);
-};
-
 export function useDetailPrefs() {
   // Three per-user detail-page toggles, each a server pref adopted on login (usePref). Default ON:
   // undefined → true, only an explicit false hides. The detail pages mount after `me` is cached, so
@@ -31,7 +20,7 @@ export function useDetailPrefs() {
   // Windows also reports whenever its animation effects are off). Once the reader turns it on or off
   // here, that pick wins on every device: the owner never saw the crossfade, because the system signal
   // used to switch it off whatever the toggle said (2026-10-03). The toggle shows the state in force.
-  const reduce = useSyncExternalStore(onReducedMotionChange, reducedMotion, () => false);
+  const reduce = useMediaQuery(REDUCED_MOTION, false);
   const [crossfadePick, setCrossfade] = usePref<boolean | null>("plexInfoArtCrossfade", null, {
     parse: (raw) => (typeof raw === "boolean" ? raw : null),
   });