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/components/MyLogViewer.tsx

@@ -51,6 +51,21 @@ function useLogEvents(): LogEvent[] {
   return list;
 }
 
+// A row's React key: the event OBJECT's own number, handed out the first time it is drawn. The ring
+// keeps the same objects, so the key stays put as the list grows at the top — an index key shifted
+// every row on every new event, remounting all of them and dropping the focus off the row button
+// (at Részletes a click on a row logs a line itself, so it happened on every click).
+const rowIds = new WeakMap<LogEvent, number>();
+let nextRowId = 0;
+const rowKey = (event: LogEvent) => {
+  let id = rowIds.get(event);
+  if (id === undefined) {
+    id = nextRowId++;
+    rowIds.set(event, id);
+  }
+  return id;
+};
+
 const matches = (event: LogEvent, needle: string) =>
   !needle ||
   [event.event, event.msg, event.request_id ?? "", event.attrs ? JSON.stringify(event.attrs) : ""]
@@ -151,11 +166,11 @@ export default function MyLogViewer() {
             {all.length === 0 ? t("settings.myLog.emptyAll") : t("settings.myLog.empty")}
           </p>
         ) : (
-          shown.slice(0, ROWS_SHOWN).map((e, i) => {
+          shown.slice(0, ROWS_SHOWN).map((e) => {
             const tag = LEVEL_TAG[e.level];
             const isOpen = open === e;
             return (
-              <div key={`${e.ts}-${i}`} className="border-b border-border/50 last:border-b-0">
+              <div key={rowKey(e)} className="border-b border-border/50 last:border-b-0">
                 <button
                   type="button"
                   aria-expanded={isOpen}