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/DiagnosticsSettings.dom.test.tsx

@@ -63,6 +63,7 @@ function mount(me: Partial<Me>) {
       <DiagnosticsSettings />
     </QueryClientProvider>,
   );
+  return qc;
 }
 
 const levelButton = (name: string) =>
@@ -113,6 +114,58 @@ describe("DiagnosticsSettings", () => {
     await waitFor(() => expect(setLevel).toHaveBeenCalledWith("INFO"));
   });
 
+  it("the focus goes back to the level picker when the box it was in goes away", async () => {
+    mount({});
+    fireEvent.click(levelButton("settings.diagnostics.levels.DEBUG"));
+    const cancel = screen.getByRole("button", { name: "settings.diagnostics.consent.cancel" });
+    cancel.focus();
+    fireEvent.click(cancel);
+    expect(document.activeElement).toBe(levelButton("settings.diagnostics.levels.INFO"));
+
+    fireEvent.click(levelButton("settings.diagnostics.levels.TRACE"));
+    const confirm = screen.getByTestId("diagnostics-consent-confirm");
+    confirm.focus();
+    fireEvent.click(confirm);
+    await waitFor(() => expect(screen.getByTestId("diagnostics-status")).toBeTruthy());
+    expect(document.activeElement).toBe(levelButton("settings.diagnostics.levels.TRACE"));
+
+    const revert = screen.getByRole("button", { name: "settings.diagnostics.revert" });
+    revert.focus();
+    fireEvent.click(revert);
+    await waitFor(() => expect(screen.queryByTestId("diagnostics-status")).toBeNull());
+    expect(document.activeElement).toBe(levelButton("settings.diagnostics.levels.INFO"));
+  });
+
+  it("the ticking status line is not a live region", async () => {
+    const until = new Date(Date.now() + 3_600_000).toISOString();
+    mount({ diagnostics: { level: "DEBUG", until, retention_days: 7 } });
+    const status = screen.getByTestId("diagnostics-status");
+    expect(status.closest('[role="status"], [aria-live]')).toBeNull();
+  });
+
+  it("a `me` fetch that left before the change cannot put the old level back", async () => {
+    const qc = mount({});
+    let answerStale: (me: unknown) => void = () => {};
+    const stale = qc
+      .fetchQuery({
+        queryKey: qk.me(),
+        queryFn: () => new Promise((resolve) => (answerStale = resolve)),
+      })
+      .catch(() => null);
+    fireEvent.click(levelButton("settings.diagnostics.levels.TRACE"));
+    fireEvent.click(screen.getByTestId("diagnostics-consent-confirm"));
+    await waitFor(() => expect(screen.getByTestId("diagnostics-status")).toBeTruthy());
+    answerStale({
+      id: 1,
+      is_demo: false,
+      preferences: {},
+      diagnostics: { level: "INFO", until: null, retention_days: 7 },
+    });
+    await stale;
+    expect(qc.getQueryData<Me>(qk.me())?.diagnostics.level).toBe("TRACE");
+    expect(screen.getByTestId("diagnostics-status")).toBeTruthy();
+  });
+
   it("a lapsed level shows as the base level", () => {
     const until = new Date(Date.now() - 60_000).toISOString();
     mount({ diagnostics: { level: "TRACE", until, retention_days: 7 } });