feat(ui): app-shell behaviour — restore validation, return-to-lobby, push

- A restored game that no longer exists (cancelled/removed/revoked) drops to
  the lobby with a toast instead of the in-game error state: game-state
  exposes a `notFound` flag and the shell redirects via appScreen.go("lobby").
- Add a visible "return to lobby" control to the in-game header.
- Push/toast deep-links use activeView.select(...) (no URL); fix a latent
  visibility-listener double-install on in-place game switches.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/src/lib/header/header.svelte

@@ -18,6 +18,7 @@ absent until Phase 24 wires push-event state.
 <script lang="ts">
 	import { getContext } from "svelte";
 	import { i18n } from "$lib/i18n/index.svelte";
+	import { appScreen } from "$lib/app-nav.svelte";
 	import {
 		GAME_STATE_CONTEXT_KEY,
 		type GameStateStore,
@@ -56,6 +57,14 @@ absent until Phase 24 wires push-event state.
 		<TurnNavigator />
 	</div>
 	<div class="right">
+		<button
+			type="button"
+			class="return-to-lobby"
+			data-testid="return-to-lobby"
+			onclick={() => appScreen.go("lobby")}
+		>
+			{i18n.t("game.shell.menu.return_to_lobby")}
+		</button>
 		<button
 			type="button"
 			class="sidebar-toggle"
@@ -101,6 +110,20 @@ absent until Phase 24 wires push-event state.
 		overflow: hidden;
 		text-overflow: ellipsis;
 	}
+	.return-to-lobby {
+		font: inherit;
+		font-size: 0.85rem;
+		padding: 0.25rem 0.6rem;
+		background: transparent;
+		color: inherit;
+		border: 1px solid var(--color-border);
+		border-radius: var(--radius-sm);
+		cursor: pointer;
+		white-space: nowrap;
+	}
+	.return-to-lobby:hover {
+		background: var(--color-surface-hover);
+	}
 	.sidebar-toggle {
 		font: inherit;
 		font-size: 1.1rem;