fix(park): /park/api/config 补顶层 mqtt_url/username/password — 兼容大屏 bootstrap 取 MQTT/API 引导

大屏 main.jsx 读 cfg.mqtt_url/mqtt_username/mqtt_password；此前仅返回嵌套 mqtt{}，
大屏回退到构建期旧地址。补顶层字段(取 server-core .env MQTT_WS_URL/凭据)。

app/park/routers.py

@@ -393,7 +393,7 @@ async def update_settings(body: SettingsBody):
 
 @router.get("/api/config")
 async def runtime_config(request: Request):
-    """大屏启动引导：下发 MQTT/语音接入地址与 tick 间隔。"""
+    """大屏启动引导：下发 MQTT/API/语音地址（顶层 mqtt_url/mqtt_username/mqtt_password 字段，兼容大屏 bootstrap）。"""
     return {
         "mqtt": {
             "host": settings.MQTT_HOST,
@@ -401,10 +401,13 @@ async def runtime_config(request: Request):
             "ws_url": settings.MQTT_WS_URL,
             "tick_interval": settings.MQTT_TICK_INTERVAL,
         },
+        # 大屏 main.jsx bootstrap 读顶层字段；缺省时回退构建期烘焙地址
+        "mqtt_url": settings.MQTT_WS_URL,
+        "mqtt_username": settings.MQTT_USERNAME or "",
+        "mqtt_password": settings.MQTT_PASSWORD or "",
         "voice_ws": settings.S2S_WS_URL if settings.S2S_ENABLED else "",
-        # 大屏前端 request() 以 `${api_base}/api/...`、`${api_base}/file/...` 拼接，
-        # 故 api_base 返回 `/park` 前缀（不含 /api），避免 /api 双写。
         "media_base": f"{request.base_url}park/file",
+        # 大屏前端 request() 以 `${api_base}/api/...`、`${api_base}/file/...` 拼接，故 api_base 返回 `/park` 前缀
         "api_base": f"{request.base_url}park",
     }