fix(player): a fullscreen button the browser cannot honour is not shown

Brave on iPadOS switches element fullscreen off, patches requestFullscreen with
a video-only stand-in and fakes fullscreenEnabled, so the Plex fullscreen button
and the YouTube docked bar's did nothing. Support is now read off
Element.prototype and the buttons are left out without it (owner). A stand-in
that returns a boolean no longer throws. A test's bare matchMedia stub no
longer outlives its describe.

frontend/src/components/PlexPlayer.tabtrap.dom.test.tsx

@@ -21,6 +21,12 @@ import { api, type PlexItemDetail } from "../lib/api";
 import { pushLayer, removeLayer } from "../lib/layerStack";
 
 beforeAll(() => {
+  // A browser that can fullscreen an element, as every desktop one can: the row's last control is
+  // the fullscreen button there (jsdom has no Fullscreen API, so the button would be left out).
+  Object.defineProperty(Element.prototype, "requestFullscreen", {
+    configurable: true,
+    value: () => Promise.resolve(),
+  });
   class Stub {
     observe() {}
     unobserve() {}