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/i18n/backendIds.contract.test.ts

@@ -232,7 +232,7 @@ describe("backend ids have their names in both languages", () => {
     expect(failDetails).toEqual(expect.arrayContaining(["facebook.login_wall"]));
     expect(goneWhats).toEqual(expect.arrayContaining(["playlist", "download", "job"]));
     expect(opaqueWhats).toEqual([]);
-    expect(youtubeReasons).toEqual(["quota", "reconnect", "unavailable"]);
+    expect(youtubeReasons).toEqual(["quota", "reconnect", "unavailable", "gone"]);
     expect(crewJobs).toEqual(
       expect.arrayContaining(["Director", "Creator", "Director of Photography"]),
     );