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/useMediaQuery.ts

@@ -1,5 +1,9 @@
 import { useSyncExternalStore } from "react";
 
+/** The reader asked the system for less motion (Windows also reports it whenever its animation
+ *  effects are off). One spelling for the hook and for a one-off `matchMedia` read alike. */
+export const REDUCED_MOTION = "(prefers-reduced-motion: reduce)";
+
 /** Whether a CSS media query matches, live. For a component that must RENDER differently at a
  *  breakpoint rather than merely hide one of two renders with CSS (`ui/DataTable` mounted its table
  *  AND its card list at every width: double the DOM, duplicate ids and keyboard targets). Where