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/game/game-shell.svelte

@@ -515,6 +515,25 @@ return to the lobby still disposes the stores via `onDestroy`.
 					orderDraft.init({ cache, gameId: activeGameId }),
 					mailStore.init({ client, cache, gameId: activeGameId }),
 				]);
+				// A restored or stale game id may point at a game that is
+				// no longer in the player's list (cancelled, removed, or
+				// access revoked). `init` flags that distinct case via
+				// `gameState.notFound` (a transient network error keeps it
+				// false and surfaces the in-game error state instead). Drop
+				// to the lobby with a toast rather than stranding the user
+				// on the in-game "not in your list" error. Leaving the
+				// `game` screen unmounts the shell, so the stores are
+				// disposed via `onDestroy`; the rest of the bootstrap
+				// (client bind, server hydration) is skipped for the dead
+				// game.
+				if (gameState.notFound) {
+					appScreen.go("lobby");
+					toast.show({
+						messageKey: "game.events.unavailable.message",
+						durationMs: 8000,
+					});
+					return;
+				}
 				galaxyClient.set(client);
 				orderDraft.bindClient(client, {
 					getCurrentTurn: () => gameState.currentTurn,