feat(design): a page's background menu on the five list pages (E28 S3e4c)

A right-click on the EMPTY area of a page opens a menu of that page's own
toolbar: the feed (sort, reshuffle, view, watch state, content, source, clear
filters), the Plex library grid (sort, watch state, clear), the channel manager
(layout, the cards' sort, sync status, edit tags, clear the tag filter, read
subscriptions), a playlist (play all, group by, reset view, save order,
undo/redo, the list's own menu) and the notification center (mark all read).
No destructive row and no backfill.

PageShell listens on the page area; lib/backgroundTarget decides empty (not
text, a control, a link, media, or a row/card with its own menu). Shift+right-
click, a touch long-press and a right-click on selected words keep the
browser's menu. ui/PageMenu registers while mounted, so a loading or error
state keeps the browser's menu too. The rows mirror each control through
ui/pageMenuItems (sortSubmenu via pickSort, choiceSubmenu, multiSubmenu).
pageMenuScope.contract.test.ts pins the five pages.

frontend/src/components/ui/pageMenuScope.contract.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+
+// docs/DESIGN.md "Background menu": which pages have one is the owner's call (2026-09-26, scope A) —
+// the list pages whose toolbar sorts, filters or switches a view: the feed, the Plex library, the
+// channel manager, a playlist and the notification center. The tab-only pages (settings, config,
+// stats, users, downloads) keep the browser's menu. A page that starts rendering one fails here until
+// that decision is taken again.
+const files = import.meta.glob(["../**/*.tsx", "!../**/*.test.*", "!../design-lab/**", "!./**"], {
+  query: "?raw",
+  import: "default",
+  eager: true,
+}) as Record<string, string>;
+
+describe("the pages with a background menu", () => {
+  it("are the five the owner chose", () => {
+    const users = Object.entries(files)
+      .filter(([, code]) => /<PageMenu\b|\busePageMenu\(/.test(code))
+      .map(([path]) => path.replace(/^\.\.\//, ""))
+      .sort();
+    expect(users).toEqual([
+      "Channels.tsx",
+      "Feed.tsx",
+      "NotificationsPanel.tsx",
+      "Playlists.tsx",
+      "PlexBrowse.tsx",
+    ]);
+  });
+});