test(ui): the px-box probe does not trip its own rule; tsc lib-safe last()

The pxBox contract test held the literal w-[150px] it probes, which the new
lint rule refuses, and stickyTop's test used Array.at, outside the tsconfig
lib. Both landed because a local lane check piped its exit code into tail.

frontend/src/components/ui/stickyTop.dom.test.tsx

@@ -22,6 +22,8 @@ vi.mock("react-i18next", async (importOriginal) => {
 import PageScroller from "./PageScroller";
 import { ColumnHead } from "./tableHeader";
 
+const last = () => fadeTopArgs[fadeTopArgs.length - 1];
+
 afterEach(() => {
   cleanup();
   fadeTopArgs.length = 0;
@@ -50,13 +52,13 @@ function page(withTable: boolean) {
 describe("the page scroller's top fade and a sticky table header", () => {
   it("is off while the header is mounted, and back on the moment it is gone", () => {
     const view = render(page(true));
-    expect(fadeTopArgs.at(-1)).toBe(false);
+    expect(last()).toBe(false);
     view.rerender(page(false)); // e.g. the Channels card layout, or a narrow DataTable
-    expect(fadeTopArgs.at(-1)).toBe(true);
+    expect(last()).toBe(true);
   });
 
   it("is on for a page with no sticky header at all", () => {
     render(page(false));
-    expect(fadeTopArgs.at(-1)).toBe(true);
+    expect(last()).toBe(true);
   });
 });