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/player/src/styles.css

@@ -197,7 +197,7 @@ body {
   background-image:
     radial-gradient(ellipse 120% 60% at 50% -10%, rgba(212, 175, 55, 0.14), transparent 55%),
     linear-gradient(rgba(0, 0, 0, 0.88), rgba(0, 0, 0, 0.88)),
-    url('./assets/images/bg.png');
+    url('./assets/images/bg.webp');
   background-repeat: no-repeat;
   background-position: center center;
   background-size: cover;