fix(ui): redirect app root to lobby/login; evict stale root service worker

- The app root ("/", i.e. /game/) rendered a dev "workspace skeleton"
  stub, and the layout guard only redirected anonymous users off it, so
  an authenticated visitor stayed on the stub. Redirect "/" to /lobby
  (authenticated) and /login (anonymous), and replace the stub with a
  minimal loading placeholder. Drop the obsolete landing-stub unit test
  (root redirect is covered by the auth-flow e2e).
- Ship a tombstone /service-worker.js on the project site so any old
  root-scoped PWA worker (from when the game lived at the origin root)
  unregisters itself instead of serving a stale cached page at the
  site origin. The game now registers its worker only under /game/.

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

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

@@ -90,7 +90,10 @@
 		}
 		if (session.status === "anonymous" && pathname !== "/login") {
 			void goto(withBase("/login"), { replaceState: true });
-		} else if (session.status === "authenticated" && pathname === "/login") {
+		} else if (
+			session.status === "authenticated" &&
+			(pathname === "/login" || pathname === "/")
+		) {
 			void goto(withBase("/lobby"), { replaceState: true });
 		}
 	});