fix(diagnostics): a pivot or Back applies the search text at once

The query read the typing-debounced search, so for 300 ms after a jump it mixed the new view's
filters with the old view's text and fetched rows neither view shows. useDebounced takes a settle key
whose change applies the value immediately.

frontend/src/lib/useDebounced.dom.test.ts

@@ -0,0 +1,35 @@
+// @vitest-environment jsdom
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { act, renderHook } from "@testing-library/react";
+
+import { useDebounced } from "./useDebounced";
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe("useDebounced", () => {
+  it("hands a value on once it stopped changing for the delay", () => {
+    vi.useFakeTimers();
+    const { result, rerender } = renderHook(({ v }) => useDebounced(v, 300), {
+      initialProps: { v: "a" },
+    });
+    rerender({ v: "ab" });
+    expect(result.current).toBe("a");
+    act(() => vi.advanceTimersByTime(300));
+    expect(result.current).toBe("ab");
+  });
+
+  it("takes the value at once when `settle` changes — a jump, not typing", () => {
+    vi.useFakeTimers();
+    const { result, rerender } = renderHook(({ v, s }) => useDebounced(v, 300, s), {
+      initialProps: { v: "", s: 0 },
+    });
+    rerender({ v: "boom", s: 1 });
+    expect(result.current).toBe("boom");
+    rerender({ v: "boomx", s: 1 });
+    expect(result.current).toBe("boom");
+    act(() => vi.advanceTimersByTime(300));
+    expect(result.current).toBe("boomx");
+  });
+});