feat(rustdesk): 优化视频编码协商和添加公共服务器支持

- 调整视频编码优先级为 H264 > H265 > VP8 > VP9，优先使用硬件编码
- 对接 RustDesk 客户端质量预设 (Low/Balanced/Best) 到 BitratePreset
- 添加 secrets.toml 编译时读取机制，支持配置公共服务器
- 默认公共服务器: rustdesk.mofeng.run:21116
- 前端 ID 服务器输入框添加问号提示，显示公共服务器信息
- 用户留空时自动使用公共服务器

src/video/stream_manager.rs

@@ -613,6 +613,14 @@ impl VideoStreamManager {
         self.webrtc_streamer.set_video_codec(codec).await
     }
 
+    /// Set bitrate preset for the shared video pipeline
+    ///
+    /// This allows external consumers (like RustDesk) to adjust the video quality
+    /// based on client preferences.
+    pub async fn set_bitrate_preset(&self, preset: crate::video::encoder::BitratePreset) -> crate::error::Result<()> {
+        self.webrtc_streamer.set_bitrate_preset(preset).await
+    }
+
     /// Publish event to event bus
     async fn publish_event(&self, event: SystemEvent) {
         if let Some(ref events) = *self.events.read().await {