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.dom.test.tsx

@@ -56,6 +56,20 @@ describe("MyLogViewer", () => {
     expect(rows()[0]).toContain("app.uncaught");
   });
 
+  it("a row keeps its place and the focus as new events arrive above it", async () => {
+    render(<MyLogViewer />);
+    const row = within(screen.getByTestId("my-log-lines")).getAllByRole("button")[0]!;
+    row.focus();
+    fireEvent.click(row); // at Részletes this click is itself a new event in real use
+    await act(async () => {
+      log.debug("ui.click", "a newer event");
+      await new Promise((r) => requestAnimationFrame(() => r(null)));
+    });
+    expect(row.isConnected).toBe(true);
+    expect(document.activeElement).toBe(row);
+    expect(row.getAttribute("aria-expanded")).toBe("true");
+  });
+
   it("filters by level group and by text, and copies only what is shown", async () => {
     render(<MyLogViewer />);
     fireEvent.click(screen.getByRole("button", { name: /settings\.myLog\.filters\.warning/ }));