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.controls.dom.test.tsx

@@ -891,3 +891,33 @@ describe("the page under the player", () => {
     expect([wrap.style.width, wrap.style.height]).toEqual(["80%", "80%"]);
   });
 });
+
+// Brave on an iPad cannot fullscreen an element (it fakes `fullscreenEnabled`), and the button did
+// nothing there. It is shown only where the browser has the API on Element.prototype.
+describe("the fullscreen button", () => {
+  const withApi = (on: boolean) => {
+    if (on)
+      Object.defineProperty(Element.prototype, "requestFullscreen", {
+        configurable: true,
+        value: () => Promise.resolve(),
+      });
+    else delete (Element.prototype as { requestFullscreen?: unknown }).requestFullscreen;
+  };
+  afterEach(() => withApi(false));
+
+  it("is there where the browser can fullscreen an element", async () => {
+    withApi(true);
+    mount({ details: { A: item("A") } });
+    await playingAt("/media/A", 100);
+    expect(screen.queryByLabelText("plex.player.fullscreen")).not.toBeNull();
+  });
+
+  it("is left out where it cannot, whatever fullscreenEnabled claims", async () => {
+    withApi(false);
+    Object.defineProperty(document, "fullscreenEnabled", { configurable: true, value: true });
+    mount({ details: { A: item("A") } });
+    await playingAt("/media/A", 100);
+    expect(screen.queryByLabelText("plex.player.fullscreen")).toBeNull();
+    delete (document as { fullscreenEnabled?: unknown }).fullscreenEnabled;
+  });
+});