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/App.tsx

@@ -3,6 +3,7 @@ import { qk } from "./lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { api, getActiveAccount, setActiveAccount, setUnauthorizedHandler } from "./lib/api";
+import { notifyActionError } from "./lib/actionErrors";
 import { setLanguage, isSupported, type LangCode } from "./i18n";
 import { clearParams } from "./lib/urlState";
 import type { Page } from "./lib/urlState";
@@ -97,7 +98,9 @@ export default function App() {
   function setPanelLayout(panel: PanelId, next: PanelLayout) {
     setPanelLayouts((prev) => ({ ...prev, [panel]: next }));
     saveLayoutLocal(panel, next);
-    api.savePrefs({ [PREF_KEY[panel]]: next }).catch(() => {});
+    api
+      .savePrefs({ [PREF_KEY[panel]]: next })
+      .catch((e: unknown) => notifyActionError(e, t("errors.writeFailed.prefs")));
   }
 
   // Collapse state for the two full-height panels (left nav + filter sidebar). Persisted to the
@@ -287,7 +290,11 @@ export default function App() {
   // Language is edited outside the Settings page (sidebar/login), so it stays instant.
   function changeLanguage(code: LangCode) {
     setLanguage(code);
-    api.savePrefs({ language: code }).catch(() => {});
+    // Also from the signed-out login page, where the save 401s — `req` marks that as handled, so
+    // the reporter stays quiet there.
+    api
+      .savePrefs({ language: code })
+      .catch((e: unknown) => notifyActionError(e, t("errors.writeFailed.prefs")));
   }
 
   // First-run: a fresh instance is in setup mode (all other routes are locked) → show the wizard.