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/PlayerModal.tsx

@@ -57,7 +57,7 @@ import {
   formatCompact,
   relativeTime,
 } from "../lib/format";
-import { fullscreenElement } from "../lib/fullscreen";
+import { canFullscreenElements, fullscreenElement } from "../lib/fullscreen";
 import { dispatch, isActivationTarget, useKeyScope } from "../lib/keymap";
 import { useBackToClose } from "../lib/history";
 import ChannelNameLink from "./ChannelNameLink";
@@ -813,13 +813,18 @@ export default function PlayerModal({
                   </Button>
                 )}
                 <span className="h-4 w-px bg-white/20" />
-                <IconButton
-                  variant="overlay"
-                  onClick={toggleTrueFullscreen}
-                  label={isFullscreen ? t("player.exitFullscreen") : t("player.fullscreen")}
-                >
-                  {isFullscreen ? <Minimize /> : <Maximize />}
-                </IconButton>
+                {/* Absent where the browser cannot fullscreen an element (Brave on an iPad): there
+                    the window-filling view is the biggest, and YouTube's own button opens the
+                    system player. */}
+                {canFullscreenElements() && (
+                  <IconButton
+                    variant="overlay"
+                    onClick={toggleTrueFullscreen}
+                    label={isFullscreen ? t("player.exitFullscreen") : t("player.fullscreen")}
+                  >
+                    {isFullscreen ? <Minimize /> : <Maximize />}
+                  </IconButton>
+                )}
                 <Button
                   variant="overlay"
                   size="sm"