feat(design): the channel menu's Tags and Priority submenus (C1, E28 S3e3b)

Tags A-Z as in-place checks (the menu stays open for the next); Priority
as the fixed steps 10 5 3 2 1 0, an off-scale current value ticked in its
place. A pick re-sorts the list at once, moving only that row
(placeByPriority, the server's order), and the table / card pager and
the focus follow it (DataTable and useCardPager take a reveal seq,
listNav.focusNavItem lands on the row's own "…", a virtualized card
revealed first). ActionMenu rows take check: many | one. The menu reads
the row fresh from the cache and keys its target by id.

frontend/src/lib/listNav.dom.test.tsx

@@ -6,7 +6,7 @@
 import { useRef, useState } from "react";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { act, cleanup, fireEvent, render, screen, waitFor } from "@testing-library/react";
-import { navItemAttrs, useListNav } from "./listNav";
+import { focusNavItem, navItemAttrs, useListNav } from "./listNav";
 import { pushLayer, removeLayer } from "./layerStack";
 
 afterEach(cleanup);
@@ -615,3 +615,24 @@ describe("useListNav · Delete (E28 S3e2: a playlist row's X became the Delete k
     expect(gone("d0")).toBe(false);
   });
 });
+
+describe("focusNavItem", () => {
+  it("hands the focus to the control `pick` finds in the item, else to the item", () => {
+    Element.prototype.scrollIntoView ??= () => {};
+    const { container } = render(<List id="a" count={3} />);
+    act(() => focusNavItem(container, 2, (item) => item.querySelector<HTMLElement>("button")));
+    expect(focused()).toBe("a2-action");
+    act(() => focusNavItem(container, 1));
+    expect(focused()).toBe("a1");
+  });
+
+  it("brings a virtualized item in first, then focuses its control", async () => {
+    Element.prototype.scrollIntoView ??= () => {};
+    const reveal = vi.fn();
+    const { container, rerender } = render(<List id="a" count={5} mounted={2} reveal={reveal} />);
+    act(() => focusNavItem(container, 4, (item) => item.querySelector<HTMLElement>("button")));
+    expect(reveal).toHaveBeenCalledWith(4);
+    rerender(<List id="a" count={5} reveal={reveal} />);
+    await waitFor(() => expect(focused()).toBe("a4-action"));
+  });
+});