fix(playlists): a refused edit is taken back on screen, and a gone list or tag is said, not swallowed

Deep-review round 4, the playlist surfaces.

- A failed move or reorder relied on the resync to put the rows back — but the server's list is
  unchanged, so the refetch comes back deep-equal, react-query keeps the same `data` object, and the
  re-seed effect keyed on it never ran: the moved row stayed gone, the rejected order stayed on
  screen. Both restore from the cached detail now.
- A failed removal puts its row back itself: when the server is unreachable the resync fails too,
  and round 3's "keep the list on a failed refetch" guard then kept showing it as removed.
- The Plex playlist page (remove / rename / delete / reorder), the add-to-playlist popover and the
  tag manager had no failure handling at all: a list or tag deleted elsewhere (a 404, now a
  structured `not_found` the request layer leaves to the caller) failed in silence and an
  optimistic removal stayed removed. Each says it and resyncs; deleting a Plex playlist that is
  already gone just leaves the page.
- Playlists' own loading/error branches use the shared `LoadingState` / `StateMessage` (the loading
  line had no live region).
- The "player stays mounted" tests count mounts: a remount ended with the same flag as staying up.

frontend/src/components/AddToPlaylist.tsx

@@ -4,6 +4,7 @@ import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { Check, ListPlus, Plus, Youtube } from "lucide-react";
 import { api, type Playlist } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { useScrollFade } from "../lib/useScrollFade";
 import { playlistName } from "../lib/playlistName";
 import { PopoverItem, PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
@@ -49,14 +50,19 @@ export default function AddToPlaylist({
     qc.invalidateQueries({ queryKey: qk.playlist() });
   }
 
+  // A failure is said, and the list refreshed either way. With no catch, a playlist deleted on
+  // another device (a 404 `not_found/playlist` — still offered, from a cache up to 30 s old) was an
+  // unhandled rejection: the tick did not change, nothing was said, the gone row stayed.
   async function toggle(pl: Playlist) {
     if (busy) return;
     setBusy(true);
     try {
       if (pl.has_video) await api.removeFromPlaylist(pl.id, videoId);
       else await api.addToPlaylist(pl.id, videoId);
-      refresh();
+    } catch (e) {
+      notifyActionError(e, t(pl.has_video ? "playlists.removeFailed" : "playlists.addFailed"));
     } finally {
+      refresh();
       setBusy(false);
     }
   }
@@ -69,8 +75,10 @@ export default function AddToPlaylist({
       const pl = await api.createPlaylist(name);
       await api.addToPlaylist(pl.id, videoId);
       setNewName("");
-      refresh();
+    } catch (e) {
+      notifyActionError(e, t("playlists.addFailed"));
     } finally {
+      refresh();
       setBusy(false);
     }
   }