fix(plex): portal the full-screen player out of the page scroller

The player is `fixed left-0 top-0 z-50` but rendered from inside the page
scroller, so it only ever covered the viewport because that scroller happened
not to be a stacking context. Giving it the edge-fade made it one, and the mask
scoped the player's z-50 inside it: the rail, the side panel and the header
painted straight over the player, and its edges faded. Measured both ways, then
fixed where every other full-screen overlay here already sits — <body>.

Same for the Suspense fallback that stands in while the player chunk loads, and
for the floating draft save bar on the Configuration page. Swept the rest: every
other `fixed` element is either App-level or already portaled.

frontend/src/components/PlexPlayer.tsx

@@ -9,6 +9,7 @@ import {
   type ReactNode,
   type WheelEvent as ReactWheelEvent,
 } from "react";
+import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import Hls from "hls.js";
@@ -944,7 +945,12 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
   // While scrubbing the head follows the finger (preview); otherwise it tracks playback.
   const headPct = scrub != null && duration > 0 ? (scrub / duration) * 100 : pct;
 
-  return (
+  // Portaled to <body>, like every other full-screen overlay here (Modal, PlayerModal, ChatDock,
+  // BackToTop). A fixed, full-viewport player must not be painted inside the page scroller: the
+  // moment that scroller gets a mask/filter/transform/contain it becomes a stacking context, and
+  // this z-50 is then scoped inside it — the rail, the side panel and the header paint straight
+  // over the player. That is exactly what the scroller's edge-fade did before this moved out.
+  return createPortal(
     <div
       ref={wrapRef}
       className="fixed left-0 top-0 z-50 bg-black flex items-center justify-center select-none"
@@ -1619,7 +1625,8 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
           </div>
         </div>
       )}
-    </div>
+    </div>,
+    document.body
   );
 }