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/chatDock.ts

@@ -2,8 +2,8 @@
 // A window can be opened from anywhere (a conversation's "pop out" button, or automatically
 // when a message arrives) and stays open across page navigation. The dock state (which chats
 // are open + their minimised state) is persisted per user so it survives a reload.
-import { useSyncExternalStore } from "react";
 import { LS } from "./storage";
+import { createStore } from "./store";
 
 export interface DockChat {
   partnerId: number;
@@ -16,9 +16,8 @@ export interface DockChat {
 type PartnerLike = { id: number; name: string; avatar_url: string | null };
 
 const MAX_OPEN = 3; // keep the dock from overflowing the corner
-let chats: DockChat[] = [];
+const dock = createStore<DockChat[]>([]);
 let userId: number | null = null;
-const subs = new Set<() => void>();
 
 function storageKey(): string | null {
   return userId != null ? LS.chatDock(userId) : null;
@@ -29,16 +28,15 @@ function persist(): void {
   if (!k) return;
   try {
     // Don't persist the transient flash counter.
-    localStorage.setItem(k, JSON.stringify(chats.map(({ flash, ...c }) => c)));
+    localStorage.setItem(k, JSON.stringify(dock.get().map(({ flash, ...c }) => c)));
   } catch {
     /* ignore quota/serialization errors */
   }
 }
 
 function emit(next: DockChat[]): void {
-  chats = next;
+  dock.set(next);
   persist();
-  for (const s of subs) s();
 }
 
 // Bind the dock to the signed-in user and restore their persisted windows (once per user).
@@ -53,15 +51,17 @@ export function initDock(uid: number): void {
   } catch {
     restored = [];
   }
-  chats = restored.slice(0, MAX_OPEN);
-  for (const s of subs) s();
+  dock.set(restored.slice(0, MAX_OPEN));
 }
 
 export function openChat(partner: PartnerLike): void {
-  const existing = chats.find((c) => c.partnerId === partner.id);
+  const existing = dock.get().find((c) => c.partnerId === partner.id);
   if (existing) {
     // Re-opening focuses it: ensure expanded and move to the front.
-    emit([{ ...existing, minimized: false }, ...chats.filter((c) => c.partnerId !== partner.id)]);
+    emit([
+      { ...existing, minimized: false },
+      ...dock.get().filter((c) => c.partnerId !== partner.id),
+    ]);
     return;
   }
   emit(
@@ -73,7 +73,7 @@ export function openChat(partner: PartnerLike): void {
         minimized: false,
         flash: 0,
       },
-      ...chats,
+      ...dock.get(),
     ].slice(0, MAX_OPEN),
   );
 }
@@ -81,7 +81,7 @@ export function openChat(partner: PartnerLike): void {
 // An incoming message arrived from `partner`: open the window if it's closed, otherwise just
 // flash it (whether expanded or minimised — don't disturb the minimised state).
 export function notifyIncoming(partner: PartnerLike): void {
-  const existing = chats.find((c) => c.partnerId === partner.id);
+  const existing = dock.get().find((c) => c.partnerId === partner.id);
   if (!existing) {
     emit(
       [
@@ -92,29 +92,22 @@ export function notifyIncoming(partner: PartnerLike): void {
           minimized: false,
           flash: 1,
         },
-        ...chats,
+        ...dock.get(),
       ].slice(0, MAX_OPEN),
     );
     return;
   }
-  emit(chats.map((c) => (c.partnerId === partner.id ? { ...c, flash: c.flash + 1 } : c)));
+  emit(dock.get().map((c) => (c.partnerId === partner.id ? { ...c, flash: c.flash + 1 } : c)));
 }
 
 export function closeChat(partnerId: number): void {
-  emit(chats.filter((c) => c.partnerId !== partnerId));
+  emit(dock.get().filter((c) => c.partnerId !== partnerId));
 }
 
 export function toggleMinimize(partnerId: number): void {
-  emit(chats.map((c) => (c.partnerId === partnerId ? { ...c, minimized: !c.minimized } : c)));
+  emit(dock.get().map((c) => (c.partnerId === partnerId ? { ...c, minimized: !c.minimized } : c)));
 }
 
 export function useDockChats(): DockChat[] {
-  return useSyncExternalStore(
-    (cb) => {
-      subs.add(cb);
-      return () => subs.delete(cb);
-    },
-    () => chats,
-    () => chats,
-  );
+  return dock.useValue();
 }