feat(ui): screen-level history for the app-shell (Back → lobby)

Mirror the screen into browser history via SvelteKit shallow routing
(pushState/replaceState with page.state) so Back/Forward move between
screens while the URL stays at /game/. Overlays (game, lobby-create) push;
lobby/login replace. A popstate→page.state effect syncs the store back
without re-pushing (no loop); the boot stamp puts a restored overlay above
the load entry so Back falls through to lobby. In-game view switches never
touch history.

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

ui/frontend/src/app.d.ts

@@ -3,7 +3,13 @@ declare global {
 	const __APP_VERSION__: string;
 
 	namespace App {
-		// future-phase types added later
+		// Shallow-routing state for the single-URL app-shell: the screen
+		// (and active game) live in `page.state` so browser Back/Forward
+		// move between screens while the address bar stays at /game/.
+		interface PageState {
+			screen?: "login" | "lobby" | "lobby-create" | "game";
+			gameId?: string | null;
+		}
 	}
 }
 

ui/frontend/src/lib/app-nav.svelte.ts

@@ -20,6 +20,8 @@
 // in the shell via SvelteKit shallow routing; this module is the source of
 // truth, history only mirrors it.
 
+import { pushState, replaceState } from "$app/navigation";
+
 export type AppScreen = "login" | "lobby" | "lobby-create" | "game";
 
 export type GameView =
@@ -158,6 +160,34 @@ class AppScreenStore {
 			this.#gameId = null;
 		}
 		persist();
+		this.#syncHistory();
+	}
+
+	/**
+	 * syncFromHistory applies a screen restored from browser history (a
+	 * Back/Forward popstate) WITHOUT pushing a new entry. An absent/unknown
+	 * screen (the load entry beneath an overlay) falls back to lobby.
+	 */
+	syncFromHistory(screen: AppScreen | undefined, gameId: string | null): void {
+		const next =
+			screen !== undefined && APP_SCREENS.includes(screen) ? screen : "lobby";
+		this.#screen = next;
+		this.#gameId = next === "game" ? gameId : null;
+		persist();
+	}
+
+	// Mirror the screen into browser history via shallow routing (the URL is
+	// unchanged — the address bar stays at /game/). Overlays (game,
+	// lobby-create) push a new entry so browser Back returns to the lobby
+	// beneath; lobby/login replace in place.
+	#syncHistory(): void {
+		if (typeof window === "undefined") return;
+		const state: App.PageState = { screen: this.#screen, gameId: this.#gameId };
+		if (this.#screen === "game" || this.#screen === "lobby-create") {
+			pushState("", state);
+		} else {
+			replaceState("", state);
+		}
 	}
 }
 

ui/frontend/src/routes/+page.svelte

@@ -12,6 +12,31 @@
 	import LobbyScreen from "$lib/screens/lobby-screen.svelte";
 	import LobbyCreateScreen from "$lib/screens/lobby-create-screen.svelte";
 	import GameShell from "$lib/game/game-shell.svelte";
+	import { pushState } from "$app/navigation";
+	import { page } from "$app/state";
+
+	// Screen-level browser history (Back → lobby) without changing the URL.
+	// On the first authenticated render, stamp a restored overlay (game /
+	// lobby-create) on top of the load entry so Back falls through to lobby.
+	let historyStamped = $state(false);
+	$effect(() => {
+		if (session.status === "authenticated" && !historyStamped) {
+			historyStamped = true;
+			if (appScreen.screen === "game" && appScreen.gameId !== null) {
+				pushState("", { screen: "game", gameId: appScreen.gameId });
+			} else if (appScreen.screen === "lobby-create") {
+				pushState("", { screen: "lobby-create" });
+			}
+		}
+	});
+
+	// Sync the store from history on Back/Forward (popstate updates
+	// `page.state`). Skipped until the baseline is stamped so it never
+	// clobbers the restored screen on first render.
+	$effect(() => {
+		if (!historyStamped) return;
+		appScreen.syncFromHistory(page.state.screen, page.state.gameId ?? null);
+	});
 </script>
 
 {#if session.status === "authenticated"}