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/lib/fullscreen.ts

@@ -8,7 +8,7 @@ import {
 
 // The browser's Fullscreen API, WebKit included.
 //
-// Safari and iPadOS still ship only the prefixed spelling: no `fullscreenchange`, no
+// Safari and iPadOS before 16.4 ship only the prefixed spelling: no `fullscreenchange`, no
 // `fullscreenElement`, no `requestFullscreen` on a plain element. Reading only the unprefixed names
 // there means the page believes it is NEVER fullscreen — which silently breaks whatever the
 // fullscreen state drives (an overlay that must yield, a focus reclaim, an Esc branch that would
@@ -29,11 +29,26 @@ export function fullscreenElement(): Element | null {
   return d.fullscreenElement ?? d.webkitFullscreenElement ?? null;
 }
 
+/** Can this browser put a plain element in fullscreen at all? Read off `Element.prototype`, never
+ * `document.fullscreenEnabled`: Brave on iPadOS switches element fullscreen off in its web view, then
+ * patches `HTMLElement.prototype.requestFullscreen` with a stand-in that only fullscreens a <video>
+ * and forces `fullscreenEnabled` to true — both claim a support that is not there, and the players'
+ * fullscreen buttons did nothing. A button that cannot work is not shown (owner). */
+export function canFullscreenElements(): boolean {
+  if (typeof Element === "undefined") return false;
+  const proto = Element.prototype as Partial<WebkitElement>;
+  return (
+    typeof proto.requestFullscreen === "function" ||
+    typeof proto.webkitRequestFullscreen === "function"
+  );
+}
+
 /** Ask for fullscreen on `el`. Errors are swallowed on purpose: a refused request (no user
  * activation, an iOS device that only fullscreens <video>) is not something a caller can fix. */
 export function requestFullscreen(el: HTMLElement): void {
   const e = el as WebkitElement;
-  if (e.requestFullscreen) void e.requestFullscreen().catch(() => {});
+  // Wrapped: a patched stand-in (Brave's, above) may return a boolean, not a promise.
+  if (e.requestFullscreen) void Promise.resolve(e.requestFullscreen()).catch(() => {});
   else e.webkitRequestFullscreen?.();
 }