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/PlexBrowse.tsx

@@ -1,4 +1,5 @@
 import { lazy, Suspense, useEffect, useLayoutEffect, useRef, useState, type CSSProperties } from "react";
+import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
 import { useInfiniteQuery, useQuery, useQueryClient, type InfiniteData } from "@tanstack/react-query";
 import {
@@ -226,8 +227,10 @@ export default function PlexBrowse({
   }
 
   if (sub.view.kind === "player") {
+    // The fallback is portaled for the same reason the player itself is: it is a fixed
+    // full-viewport overlay, and this tree renders inside the page scroller.
     return (
-      <Suspense fallback={<div className="fixed inset-0 z-50 bg-black" />}>
+      <Suspense fallback={createPortal(<div className="fixed inset-0 z-50 bg-black" />, document.body)}>
         <PlexPlayer itemId={sub.view.id} queue={sub.view.queue} onClose={sub.back} />
       </Suspense>
     );