fix: PNG cover 404 via nginx + clear upload residual after submit

- Fix UAT nginx static cache rule intercepting /api/files/*.png requests,
  use negative lookahead to exclude /api/ paths (same as Dev config)
- Clear el-upload file list after successful game creation to prevent
  residual file path showing in next upload

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/nginx.uat.conf

@@ -60,8 +60,8 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
     }
 
-    # 静态资源缓存
-    location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
+    # 静态资源缓存（排除 /api/ 路径，避免拦截 PB 文件请求）
+    location ~* ^/(?!api/|voice-api/).*\.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot|webp)$ {
         expires 1y;
         add_header Cache-Control "public, immutable";
     }