test(ui): assert relative manifest start_url in the PWA spec

The single-origin manifest now uses relative URLs (`start_url: "./"`) so
it stays base-agnostic under `/` and `/game/`. Update the PWA spec to
assert the relative value instead of the old absolute `/`.

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

ui/frontend/tests/pwa/pwa.spec.ts

@@ -18,7 +18,10 @@ test.describe("PWA", () => {
 			.get(href!)
 			.then((r) => r.json());
 		expect(manifest.name).toBe("Galaxy");
-		expect(manifest.start_url).toBe("/");
+		// Relative so the manifest is base-agnostic: served under the app
+		// base (`/` at the root, `/game/` in the single-origin deploy) it
+		// resolves to the app root either way.
+		expect(manifest.start_url).toBe("./");
 		expect(manifest.display).toBe("standalone");
 		const sizes = (manifest.icons as { sizes: string }[]).map((i) => i.sizes);
 		expect(sizes).toContain("192x192");