feat(ui): app-shell core — single-route dispatcher, route collapse, nav→state

Collapse the game UI to one route (`/`): a screen dispatcher renders
login/lobby/lobby-create/game from `appScreen`/`activeView` state instead of
URL routes. Move screen components to lib/screens & lib/game; the game shell
reads the game id from `appScreen.gameId` and re-inits per-game stores via an
$effect; in-game views render from `activeView`. Flip ~23 goto/href nav sites
to store mutations; drop the `?sidebar=` URL coupling. Auth gate is now
state-based. WIP: browser-history (Back→lobby), restore-validation, the
return-to-lobby button, push deep-links, and the test migration are follow-ups
on this branch.

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

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

@@ -1,18 +1,32 @@
 <script lang="ts">
-	// The app root renders no content of its own. The root layout's auth
-	// guard redirects "/" to /lobby (authenticated) or /login
-	// (anonymous); this placeholder only shows for the brief moment
-	// before that client-side redirect resolves.
-	import { i18n } from "$lib/i18n/index.svelte";
+	// Single-route screen dispatcher for the app-shell. There are no
+	// per-screen routes: the visible screen is selected from in-memory
+	// state (`session.status` for the auth gate, `appScreen.screen` for
+	// the authenticated screen) rather than from the URL. The root
+	// layout intercepts the `loading` and `unsupported` session states
+	// before this component renders, so here `session.status` is either
+	// `anonymous` (login) or `authenticated` (lobby / create / game).
+	import { session } from "$lib/session-store.svelte";
+	import { appScreen } from "$lib/app-nav.svelte";
+	import LoginScreen from "$lib/screens/login-screen.svelte";
+	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";
 </script>
 
-<main class="status">
-	<p>{i18n.t("common.loading")}</p>
-</main>
-
-<style>
-	.status {
-		padding: var(--space-6);
-		font-family: var(--font-sans);
-	}
-</style>
+{#if session.status === "authenticated"}
+	{#if appScreen.screen === "lobby-create"}
+		<LobbyCreateScreen />
+	{:else if appScreen.screen === "game" && appScreen.gameId !== null}
+		<GameShell />
+	{:else}
+		<!--
+		Default authenticated screen. Covers `lobby`, a stale `login`
+		screen restored from a previous anonymous session, and a `game`
+		screen with no active game id (a snapshot that lost its id).
+		-->
+		<LobbyScreen />
+	{/if}
+{:else}
+	<LoginScreen />
+{/if}