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/UploadDialog.dom.test.tsx

@@ -20,7 +20,11 @@ vi.mock("react-i18next", async (importOriginal) => {
 });
 vi.mock("../lib/api", () => ({ api: { uploadDownload: h.uploadDownload } }));
 vi.mock("../lib/downloads", () => ({ invalidateDownloads: vi.fn() }));
-vi.mock("../lib/notifications", () => ({ notify: h.notify }));
+// The rest of the module stays real: the dialog is a ui/Modal, which hosts its own toasts.
+vi.mock("../lib/notifications", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("../lib/notifications")>()),
+  notify: h.notify,
+}));
 vi.mock("../lib/nav", () => ({ navigateTo: vi.fn() }));
 
 import UploadDialog from "./UploadDialog";