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.tsx

@@ -70,7 +70,7 @@ import {
   usePlexTracks,
   useWallClock,
 } from "../lib/plexPlayerHooks";
-import { fullscreenElement } from "../lib/fullscreen";
+import { canFullscreenElements, fullscreenElement } from "../lib/fullscreen";
 import {
   dispatch,
   isActivationTarget,
@@ -1632,9 +1632,13 @@ export default function PlexPlayer({ itemId, onClose, queue, fromStart = false }
                 />
               )}
             </div>
-            <Ctrl label={t("plex.player.fullscreen")} onClick={toggleFs} align="end">
-              {fs ? <Minimize className="w-5 h-5" /> : <Maximize className="w-5 h-5" />}
-            </Ctrl>
+            {/* Absent where the browser cannot fullscreen an element (Brave on an iPad): the button
+                did nothing there, and the windowed player already fills the window. */}
+            {canFullscreenElements() && (
+              <Ctrl label={t("plex.player.fullscreen")} onClick={toggleFs} align="end">
+                {fs ? <Minimize className="w-5 h-5" /> : <Maximize className="w-5 h-5" />}
+              </Ctrl>
+            )}
           </div>
         </div>
       </div>