fix(ui): a page's sub-tab is remembered per browser tab, not forever

Settings, Configuration, Statistics and Users kept the last sub-tab in localStorage, so every
fresh tab opened Configuration on "Plex" as if that were its home. useTabPersistedState keeps it
in sessionStorage (account-scoped): a reload and a trip to another page keep it, a new tab starts
on the first sub-tab. The pre-selects (a Review link, the Google-link return) write the same store;
a value an older build left in localStorage is cleared on first read.

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

@@ -0,0 +1,45 @@
+// @vitest-environment jsdom
+//
+// E31 (the owner): a page's sub-tab was remembered forever in localStorage, so every fresh browser tab
+// opened Configuration on "Plex" as if that were its home. The position is now kept per browser tab
+// (sessionStorage): it survives a reload and a trip to another page, and a new tab starts on the first.
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { act, renderHook } from "@testing-library/react";
+import { ACTIVE_ACCOUNT_KEY } from "./activeAccount";
+import { setTabRaw, useTabPersistedState } from "./storage";
+
+beforeEach(() => {
+  sessionStorage.setItem(ACTIVE_ACCOUNT_KEY, "1");
+});
+afterEach(() => {
+  sessionStorage.clear();
+  localStorage.clear();
+});
+
+describe("a page's sub-tab, per browser tab", () => {
+  it("starts on the fallback, then keeps the picked tab for this browser tab only", () => {
+    const first = renderHook(() => useTabPersistedState("siftlode.configTab", "access"));
+    expect(first.result.current[0]).toBe("access");
+    act(() => first.result.current[1]("plex"));
+    expect(sessionStorage.getItem("siftlode.configTab.1")).toBe("plex");
+    expect(localStorage.getItem("siftlode.configTab.1")).toBeNull();
+    // The page mounts again in this tab (a reload, or back from another page): it is where it was.
+    first.unmount();
+    expect(
+      renderHook(() => useTabPersistedState("siftlode.configTab", "access")).result.current[0],
+    ).toBe("plex");
+  });
+
+  it("ignores a position left in localStorage by an older build, and clears it", () => {
+    localStorage.setItem("siftlode.configTab.1", "plex");
+    const { result } = renderHook(() => useTabPersistedState("siftlode.configTab", "access"));
+    expect(result.current[0]).toBe("access");
+    expect(localStorage.getItem("siftlode.configTab.1")).toBeNull();
+  });
+
+  it("a pre-selected tab is the one the page opens on", () => {
+    setTabRaw("siftlode.adminUsersTab", "access");
+    const { result } = renderHook(() => useTabPersistedState("siftlode.adminUsersTab", "roles"));
+    expect(result.current[0]).toBe("access");
+  });
+});