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-state.svelte.ts

@@ -118,6 +118,17 @@ export class GameStateStore {
 	 */
 	mapToggles: MapToggles = $state({ ...DEFAULT_MAP_TOGGLES });
 	error: string | null = $state(null);
+	/**
+	 * notFound is the distinct "this game is not in the player's list"
+	 * signal, set when `findGame` returns null (cancelled, removed, or
+	 * access revoked). It is a clean flag the app-shell reads after
+	 * `init` to drop a restored/stale game back to the lobby with a
+	 * toast, rather than string-matching the `error` message. Transient
+	 * network failures keep `notFound` false (they take the catch path)
+	 * so they still surface the in-game error state for a retry. Reset
+	 * to false at the start of every `setGame` / `advanceToPending`.
+	 */
+	notFound = $state(false);
 	/**
 	 * currentTurn mirrors the engine's turn number for the running
 	 * game (lifted from the lobby record on `setGame`). Phase 14
@@ -218,6 +229,7 @@ export class GameStateStore {
 		this.gameId = gameId;
 		this.status = "loading";
 		this.error = null;
+		this.notFound = false;
 		this.report = null;
 
 		this.wrapMode = await readWrapMode(this.cache, gameId);
@@ -229,6 +241,7 @@ export class GameStateStore {
 			if (summary === null) {
 				this.status = "error";
 				this.error = `game ${gameId} is not in your list`;
+				this.notFound = true;
 				return;
 			}
 			this.gameName = summary.gameName;
@@ -306,11 +319,13 @@ export class GameStateStore {
 		}
 		this.status = "loading";
 		this.error = null;
+		this.notFound = false;
 		try {
 			const summary = await this.findGame(this.gameId);
 			if (summary === null) {
 				this.status = "error";
 				this.error = `game ${this.gameId} is not in your list`;
+				this.notFound = true;
 				return;
 			}
 			this.gameName = summary.gameName;
@@ -558,6 +573,18 @@ export class GameStateStore {
 
 	private installVisibilityListener(): void {
 		if (typeof document === "undefined") return;
+		// Idempotent on re-init. In the single-URL app-shell a direct
+		// game → game switch (a push deep-link or a refresh-restore)
+		// re-runs `init` without unmounting the shell, so a naive
+		// `addEventListener` here would stack a second listener on every
+		// switch. Drop any previously-registered one first so exactly one
+		// stays live.
+		if (this.visibilityListener !== null) {
+			document.removeEventListener(
+				"visibilitychange",
+				this.visibilityListener,
+			);
+		}
 		const listener = (): void => {
 			if (document.visibilityState === "visible" && this.status === "ready") {
 				void this.refresh();