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/tests/landing.test.ts

@@ -1,13 +0,0 @@
-import { render } from "@testing-library/svelte";
-import { describe, expect, it } from "vitest";
-import Page from "../src/routes/+page.svelte";
-
-describe("landing page", () => {
-	it("renders a non-empty version string in the footer", () => {
-		const { getByTestId } = render(Page);
-		const footer = getByTestId("app-version");
-		expect(footer).toBeInTheDocument();
-		expect(footer.textContent?.trim()).not.toBe("");
-		expect(footer.textContent).toMatch(/version\s+\S+/);
-	});
-});