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/PlexPlaylistView.tsx

@@ -31,7 +31,8 @@ import {
   Trash2,
   X,
 } from "lucide-react";
-import { api, type PlexCard } from "../lib/api";
+import { api, HttpError, type PlexCard } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { formatRuntime } from "../lib/format";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 import { useConfirm } from "./ConfirmProvider";
@@ -401,6 +402,17 @@ export default function PlexPlaylistView({
     qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
   };
 
+  // Every edit here is optimistic on the local copy, and none of them used to handle a failure: a
+  // removal that failed stayed removed on screen (the resync never ran), and a 404 — the playlist
+  // deleted on another device, now a structured `not_found` — was an unhandled rejection that said
+  // nothing at all (deep-review round 4; the YouTube side was fixed in round 3). A failure is said,
+  // and the rows are put back from what the server last sent — explicitly, because a resync that
+  // returns the same list keeps the same `q.data` and the re-sync effect above never runs.
+  const putBack = () => {
+    if (q.data) setItems(q.data.items);
+    invalidate();
+  };
+
   function commit(next: PlexCard[]) {
     setItems(next);
     api
@@ -408,7 +420,11 @@ export default function PlexPlaylistView({
         playlistId,
         next.map((i) => i.id),
       )
-      .then(() => qc.invalidateQueries({ queryKey: qk.plexPlaylists() }));
+      .then(() => qc.invalidateQueries({ queryKey: qk.plexPlaylists() }))
+      .catch((e: unknown) => {
+        notifyActionError(e, t("plex.playlist.reorderFailed"));
+        putBack();
+      });
   }
 
   async function onRemove(keys: string[]) {
@@ -420,6 +436,9 @@ export default function PlexPlaylistView({
       if (keys.length === 1) await api.plexPlaylistRemoveItem(playlistId, keys[0]!);
       else await api.plexPlaylistRemoveBulk(playlistId, keys);
       invalidate();
+    } catch (e) {
+      notifyActionError(e, t("plex.playlist.removeFailed"));
+      putBack();
     } finally {
       setBusy(false);
     }
@@ -428,7 +447,12 @@ export default function PlexPlaylistView({
   async function rename() {
     const title = name.trim();
     if (!title) return;
-    await api.plexRenamePlaylist(playlistId, title);
+    try {
+      await api.plexRenamePlaylist(playlistId, title);
+    } catch (e) {
+      notifyActionError(e, t("plex.playlist.renameFailed")); // the field stays open to try again
+      return;
+    }
     setRenaming(false);
     invalidate();
   }
@@ -440,7 +464,16 @@ export default function PlexPlaylistView({
       }))
     )
       return;
-    await api.plexDeletePlaylist(playlistId);
+    try {
+      await api.plexDeletePlaylist(playlistId);
+    } catch (e) {
+      // Gone already (deleted on another device): what was asked for is true — leave the dead page.
+      // Anything else is a real failure, and the page stays.
+      if (!(e instanceof HttpError && e.status === 404)) {
+        notifyActionError(e, t("plex.playlist.deleteFailed"));
+        return;
+      }
+    }
     qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
     onBack();
   }