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/Channels.tsx

@@ -9,6 +9,7 @@ import {
 } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
+import { createStore, type Store } from "../lib/store";
 import { qk } from "../lib/queryKeys";
 import { Ban, Eye, EyeOff, History, LayoutGrid, Pencil, RefreshCw, Table, X } from "lucide-react";
 import ExcludedChannelList from "./ExcludedChannelList";
@@ -91,24 +92,8 @@ const channelKey = (c: ManagedChannel) => c.id;
 // Which channel's row menu is open, for its "…" (`aria-expanded`, and staying shown while open). A
 // tiny store rather than a context: a context change walks the whole table to find its readers (~20
 // ms on 400 rows, each open and close), while a store calls its subscribers and re-renders only the
-// two buttons whose answer changed.
-function createOpenRowStore() {
-  let open: string | null = null;
-  const listeners = new Set<() => void>();
-  return {
-    get: () => open,
-    set: (next: string | null) => {
-      if (next === open) return;
-      open = next;
-      listeners.forEach((l) => l());
-    },
-    subscribe: (l: () => void) => {
-      listeners.add(l);
-      return () => void listeners.delete(l);
-    },
-  };
-}
-type OpenRowStore = ReturnType<typeof createOpenRowStore>;
+// two buttons whose answer changed (each reads a boolean, so a change elsewhere renders nothing).
+type OpenRowStore = Store<string | null>;
 
 function RowMoreButton({
   id,
@@ -349,7 +334,7 @@ export default function Channels() {
   // stays one line and the trailing "…" actions can't wrap.
   const tagChips = tagFilterChips({
     userTags,
-    allTags,
+    tags: tagsQuery,
     picked: tagFilter,
     noTagOn,
     toggle: toggleTagFilter,
@@ -397,7 +382,7 @@ export default function Channels() {
   // reposition, and so re-rendered the whole table — ~0.8 s to open the menu on 400 rows.
   const { buttonProps: rowButtonProps, triggerProps: rowTriggerProps } = rowMenu;
   const openRowId = rowMenu.open ? (rowMenu.target?.id ?? null) : null;
-  const [openRow] = useState(createOpenRowStore);
+  const [openRow] = useState(() => createStore<string | null>(null));
   useEffect(() => openRow.set(openRowId), [openRow, openRowId]);
   const channelActions = useChannelActions({
     onView,