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");
+  });
+});

frontend/src/lib/useDebounced.ts

@@ -3,11 +3,21 @@ import { useEffect, useState } from "react";
 // Returns `value` after it has stopped changing for `delay` ms. Lets a text input stay
 // responsive (it owns the immediate value) while the query it drives only re-runs once the
 // user pauses — so we don't refetch the feed on every keystroke.
-export function useDebounced<T>(value: T, delay = 300): T {
+//
+// `settle`: a change of it takes `value` at once. A value that did not come from typing (Back
+// restoring a search, a pivot clearing one) has no keystrokes to wait out, and the lag only ran a
+// query mixing the new filters with the old text (deep-review 2026-09-19).
+export function useDebounced<T>(value: T, delay = 300, settle?: unknown): T {
   const [debounced, setDebounced] = useState(value);
+  const [settled, setSettled] = useState(settle);
   useEffect(() => {
     const id = window.setTimeout(() => setDebounced(value), delay);
     return () => window.clearTimeout(id);
   }, [value, delay]);
+  if (settle !== settled) {
+    setSettled(settle);
+    setDebounced(value);
+    return value;
+  }
   return debounced;
 }