fix(ui): every full-screen overlay hosts its own toasts, above the players' controls, without re-announcing them

Deep-review round 3 of bug hunt 2026-09 (the round-2 overlay toast host, finished).

- Inside an overlay the toast stack sits above the bottom band (bottom-24, the UpNextCard rung) and
  lets clicks between cards through: at the app's own spot it covered the Plex transport row and
  PlayerModal's docked bar and took their clicks while a toast was up (measured: play/pause, ±10,
  stop, prev/next, mute all hit the toast).
- ui/Modal and the onboarding wizard mount an overlay host too, so a toast raised under any dialog
  is readable and a click on it no longer closes the dialog; the Modal's Tab trap spans the whole
  dialog, so a toast's action and dismiss are keyboard-reachable. A test now enforces the
  convention: a component that covers the viewport at z-overlay must mount <Toaster overlay />.
- PlayerModal's host moves into the stage in true fullscreen (outside the element holding the lock it
  was painted under it).
- A host that takes over shows the toasts it inherits silently (no second role=alert, its region
  aria-live=off until they are in), and each countdown bar starts as far in as its toast already is
  instead of restarting while the real dismiss timer does not.
- The overlay-host registry uses lib/store instead of a hand-rolled listener triad.

frontend/src/components/PlayerModal.tsx

@@ -586,6 +586,7 @@ export default function PlayerModal({
               the exit button also re-arms our shortcuts. Add-to-playlist works in true fullscreen (its
               popover portals INTO the fullscreen element); download stays hidden there because it opens
               a Modal, which still portals to <body> — outside the fullscreen element. */}
+            {isFullscreen && <Toaster overlay />}
             {(maximized || isFullscreen) && (
               <div
                 onClick={(e) => e.stopPropagation()}
@@ -962,8 +963,9 @@ export default function PlayerModal({
         )}
       </div>
       {/* Toasts raised while the player is open (a failed Remove, a lost connection) — the app's own
-          Toaster is underneath this scrim. See ui/Toaster. */}
-      <Toaster overlay />
+          Toaster is underneath this scrim. See ui/Toaster. In true fullscreen it moves into the
+          stage, the element holding the lock: out here it was painted under it, i.e. not at all. */}
+      {!isFullscreen && <Toaster overlay />}
     </div>,
     document.body,
   );