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/+layout.svelte

@@ -2,10 +2,8 @@
 	import "$lib/theme/tokens.css";
 	import "$lib/theme/base.css";
 	import { onMount } from "svelte";
-	import { goto } from "$app/navigation";
-	import { page } from "$app/state";
 	import { dev } from "$app/environment";
-	import { appBase, withBase } from "$lib/paths";
+	import { withBase } from "$lib/paths";
 	import { i18n } from "$lib/i18n/index.svelte";
 	import { session } from "$lib/session-store.svelte";
 	import { eventStream } from "../api/events.svelte";
@@ -77,25 +75,6 @@
 			eventStream.stop();
 			streamSessionId = null;
 		}
-
-		// page.url.pathname includes the configured base path; strip it so
-		// the route comparisons below stay base-agnostic.
-		const pathname = page.url.pathname.slice(appBase.length);
-		// Debug-only routes under /__debug/* run their own bootstrap
-		// path against the storage primitives and must bypass the
-		// auth guard so Phase 6's Playwright spec can drive the
-		// keystore directly.
-		if (pathname.startsWith("/__debug/")) {
-			return;
-		}
-		if (session.status === "anonymous" && pathname !== "/login") {
-			void goto(withBase("/login"), { replaceState: true });
-		} else if (
-			session.status === "authenticated" &&
-			(pathname === "/login" || pathname === "/")
-		) {
-			void goto(withBase("/lobby"), { replaceState: true });
-		}
 	});
 </script>