fix(ui): toasts raised while a full-screen player is open show inside it, not under its scrim

The app's Toaster lives in the content column, which is a stacking context (its `@container` implies
layout containment), so both full-screen players — portalled to <body> at the same z-index — painted
over every toast. Round 1's "a failed Remove is reported" fix was therefore still silent inside the
player: its toast sat under an 80% black, blurred scrim (elementFromPoint returned the scrim), and a
click on it closed the player.

An overlay now mounts its own `<Toaster overlay />`, and exactly one host shows the toasts: the most
recently mounted overlay, else the app's. Same component, same look — and inside the aria-modal
dialog, where a screen reader can still reach it. A click on a toast inside an overlay stays there.
Both players get one (the Plex player's also works in native fullscreen, where nothing outside the
fullscreen element is drawn). Verified on localdev: the removal failure reads above the scrim,
bottom-left, and clicking it leaves the player open.

frontend/src/components/PlexPlayer.tsx

@@ -43,6 +43,7 @@ import { PlexPlayerSettings } from "./PlexPlayerSettings";
 import { PlexStatsOverlay } from "./PlexStatsOverlay";
 import { createFragMeter } from "../lib/plexStats";
 import { UpNextCard } from "./UpNextCard";
+import Toaster from "./ui/Toaster";
 import {
   useAutoSkip,
   useControlsVisibility,
@@ -1379,6 +1380,9 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
           </div>
         </div>
       )}
+      {/* The app's Toaster is underneath this full-screen player (and outside it in native
+          fullscreen), so toasts raised while watching show here instead. See ui/Toaster. */}
+      <Toaster overlay />
     </div>,
     document.body,
   );