test(fe): cover the palette's content, AltGr, the banner's name, share colours, playlist headers

- AppCommandPalette had no test: modules gated by the account, hidden channels left out, the
  playlist row selecting before it navigates, and the surprise riding setPage's `then`.
- keymap: an Alt/AltGr-modified key (Ctrl+Alt on Windows, a constant on the HU layout) fires no
  bare-key or Mod row.
- Banner: the dismiss button falls back to "Close" rather than going unnamed.
- ShareDialog: a protected link's lit lock and Revoke's danger colour stay on the icon.
- Both playlist headers are rendered now: the YouTube one needed the toolbar slot it portals into.
  Meta line sums known runtimes and treats an ABSENT duration as nothing (null never exercised the
  guard: `3000 + null` is 3000); a one-row list offers no view controls.

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

@@ -0,0 +1,31 @@
+// @vitest-environment jsdom
+import { describe, it, expect, afterEach, vi } from "vitest";
+import { render, screen, cleanup } from "@testing-library/react";
+import { Info } from "lucide-react";
+
+vi.mock("react-i18next", () => ({ useTranslation: () => ({ t: (k: string) => k }) }));
+
+import Banner from "./Banner";
+
+afterEach(cleanup);
+
+// The dismiss button is an icon, so its name is all a screen reader gets. A caller that forgets
+// `dismissTitle` must still ship a NAMED control — the generic "Close" — never an unnamed one.
+describe("Banner · the dismiss button's name", () => {
+  it("falls back to the generic Close, and takes the caller's name when given", () => {
+    render(
+      <Banner tone="accent" icon={Info} onDismiss={() => {}}>
+        News
+      </Banner>,
+    );
+    expect(screen.getByRole("button", { name: "common.close" })).toBeTruthy();
+    cleanup();
+
+    render(
+      <Banner tone="accent" icon={Info} onDismiss={() => {}} dismissTitle="Hide the notes">
+        News
+      </Banner>,
+    );
+    expect(screen.getByRole("button", { name: "Hide the notes" })).toBeTruthy();
+  });
+});