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/ui/Toaster.hosts.test.ts

@@ -0,0 +1,33 @@
+// Every full-screen overlay hosts its own toasts (see ui/Toaster).
+//
+// The app's Toaster lives in the content column, a stacking context: anything that covers the
+// viewport at z-overlay paints over it, so a toast raised while that overlay is open is unreadable and
+// a click on it lands on the overlay's backdrop. The fix is a convention — the overlay mounts
+// `<Toaster overlay />` inside itself — and a convention stated in a comment is one the next overlay
+// forgets (deep-review round 3 found every ui/Modal and the onboarding wizard without one). So it is
+// checked here: a component that renders a viewport-covering z-overlay layer mounts an overlay host.
+import { describe, it, expect } from "vitest";
+
+const sources = import.meta.glob("../**/*.tsx", { query: "?raw", import: "default", eager: true });
+
+// Covers the viewport for a moment with nothing a toast could be about (a lazy chunk's black
+// placeholder while the player loads).
+const EXEMPT = new Set(["../PlexBrowse.tsx"]);
+
+describe("full-screen overlays host their own toasts", () => {
+  it("every one that covers the viewport at z-overlay mounts <Toaster overlay />", () => {
+    const missing = Object.entries(sources)
+      .filter(([path]) => !path.endsWith(".test.tsx") && !EXEMPT.has(path))
+      .filter(([, src]) => /fixed inset-0 z-overlay/.test(src as string))
+      .filter(([, src]) => !(src as string).includes("<Toaster overlay />"))
+      .map(([path]) => path);
+    expect(missing).toEqual([]);
+  });
+
+  it("finds the overlays it is meant to check", () => {
+    const covered = Object.entries(sources).filter(([, src]) =>
+      /fixed inset-0 z-overlay/.test(src as string),
+    );
+    expect(covered.length).toBeGreaterThanOrEqual(3); // Modal, PlayerModal, the wizard at least
+  });
+});