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/locales/hu/common.json

@@ -26,6 +26,7 @@
   "retry": "Újra",
   "retryWithEnter": "Újrapróbáláshoz nyomd meg az Entert.",
   "loadError": "Nem sikerült betölteni — ellenőrizd a kapcsolatot, és próbáld újra",
+  "deletedTag": "Törölt címke",
   "seeAll": "Összes",
   "scrollLeft": "Görgetés balra",
   "scrollRight": "Görgetés jobbra",