fix: the diagnostics tab keeps its focus, its level and its consent

Deep-review round 1 of E22.

- After a 429 the stream sent one POST per new event while a full batch
  waited; it now stays quiet for 15 s. Past the queue cap the oldest
  events go, as the comment always said, a refused batch included.
- A batch refused after the level was lowered came back into the queue
  of a later consent in the same tab; each consent is its own session.
- A `me` fetch that left before a level or preference change could put the
  old value back for up to a minute. `updateMeCache` writes the cache and
  cancels that fetch; usePref goes through it too.
- The consent box's buttons and "revert now" took the focus with them to
  <body>; it returns to the level picker. Watched until the button is
  gone — in Chrome the saved level arrives in two renders, and a one-shot
  check spent itself on the first (jsdom never showed it).
- The countdown line is no longer a live region re-read every minute.
- "Saját napló" rows were keyed by index on a newest-first list, so every
  new event remounted every row and dropped the focus off a row button.

frontend/src/lib/usePref.ts

@@ -5,6 +5,7 @@ import { api } from "./api";
 import type { Me } from "./api/account";
 import { readAccount, writeAccount } from "./storage";
 import { notifyActionError } from "./actionErrors";
+import { updateMeCache } from "./useMe";
 import i18n from "../i18n";
 
 // The local state + cache already show the new value, so a refused save is otherwise invisible until
@@ -45,9 +46,7 @@ function readServerPref(me: Me | null | undefined, key: string): unknown {
 // (their sole difference is WHEN the server write fires — immediately vs debounced), so the cache-merge
 // shape lives in ONE place.
 function mergePrefsCache(qc: QueryClient, patch: Record<string, unknown>): void {
-  qc.setQueryData<Me | null>(qk.me(), (m) =>
-    m ? { ...m, preferences: { ...m.preferences, ...patch } } : m,
-  );
+  updateMeCache(qc, (m) => ({ ...m, preferences: { ...m.preferences, ...patch } }));
 }
 
 /** The raw preference persister: save an arbitrary partial preferences patch to the server and merge