fix(ui): a write that fails says what failed and why, wherever it fails

`req` opens its blocking dialog for 400/409/422/5xx only. A 403/404/429 was left to the caller and a
502-504 or lost connection got just the self-clearing connection notice, so 34 mutations without a
reporting onError and 15 direct api writes with a bare or missing catch failed in silence: a hidden
channel that snapped back, a share link that never revoked, a confirmed account delete that ended
with nothing said.

- lib/queryClient: one MutationCache reporter. react-query awaits it BEFORE the mutation's own
  onError, so it waits a task and skips a failure the site already reported; the toast title comes
  from `meta.failTitle`, `meta.errorShownInline` opts out.
- notifyActionError: title names WHAT failed, the line under it says WHY (`failureReason`: a
  structured localized detail, else the status). It marks the error reported (`markReported`), so
  nothing reports one failure twice; `notifyKnownFailure` for a site's own specific sentence.
- Every silent site named (45 new HU/EN titles, 4 reasons). Stale "the global dialog shows it"
  comments corrected on the job actions, channel block, Plex sync, cookies, download enqueue,
  account delete and usePref.
- Channels passed the tag hook's channel id to the reporter as the error; the tag and priority hooks
  now report themselves, with the real error.
- A contract test keeps every useMutation naming its failure; CLAUDE.md gains the pattern row.
- history.dom.test: the "navigates only once the entries are back" assertion raced an async act's
  macrotask flush (1 in 5 runs); it now asserts from a later popstate listener.

frontend/src/components/PlexSidebar.tsx

@@ -6,6 +6,7 @@ import { plexLang } from "../i18n";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { ListMusic, Plus, SlidersHorizontal, X } from "lucide-react";
 import { api, plexFilterCount, type PlexFilters } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { type PanelLayout } from "../lib/panelLayout";
 import { type MediaSourceKind } from "../lib/mediaSource";
 import { useDebounced } from "../lib/useDebounced";
@@ -377,9 +378,14 @@ function PlaylistPicker() {
   ];
 
   async function createPlaylist(title: string) {
-    const pl = await api.plexCreatePlaylist(title);
-    qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
-    onOpenPlaylist(pl.id);
+    try {
+      const pl = await api.plexCreatePlaylist(title);
+      qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
+      onOpenPlaylist(pl.id);
+    } catch (e) {
+      // Called without an await from the picker, so a refusal was an unhandled rejection (E11 S2b).
+      notifyActionError(e, t("plex.playlistAdd.createFailed"));
+    }
   }
 
   return (