perf(assets): Player/Admin 静态图改用 WebP，vs 保留 PNG

Player 端 banner/bg/card/h5bg 等改为 webp 并移除旧 png；Admin 登录背景改 webp；vs 对阵图仍用 png。

Co-authored-by: Cursor <cursoragent@cursor.com>

apps/admin/src/views/Login.vue

@@ -7,7 +7,7 @@ import { useAuthStore, type StaffUser } from '../stores/auth';
 import RobotVerify from '../components/RobotVerify.vue';
 import AdminLocaleSwitcher from '../components/AdminLocaleSwitcher.vue';
 import { useAdminLocale } from '../composables/useAdminLocale';
-import bgImage from '../assets/images/bg.png';
+import bgImage from '../assets/images/bg.webp';
 
 const router = useRouter();
 const route = useRoute();