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/components/PlexSidebar.tsx

@@ -439,7 +439,7 @@ function CollectionPicker({
   // …but a selection restored from a LINK has ids and no titles, and the list above is only fetched
   // once the picker is opened — so the token rendered the raw rating_key until then. Resolve just the
   // selected ids (see usePlexCollectionTitles).
-  const resolved = usePlexCollectionTitles(values, titles);
+  const { titles: resolved, pending } = usePlexCollectionTitles(values, titles);
   const options: ComboboxOption[] = (q.data?.collections ?? []).map((c) => ({
     value: c.id,
     label: c.title,
@@ -459,6 +459,7 @@ function CollectionPicker({
         options={options}
         selected={values}
         selectedLabels={resolved}
+        labelsPending={pending}
         query={search}
         onQueryChange={setSearch}
         onOpenChange={setOpen}