refactor: 清理死代码和优化日志级别

- 删除未使用的函数和常量
  - create_public_key_message (rustdesk/connection)
  - decode_audio_packet, AudioPacketHeader (web/audio_ws)
  - io_error_to_hid_error, close_device, close_all_devices (hid)
  - shutdown_rx (rustdesk/mod)
  - CONNECT_TIMEOUT_MS, RESP_ERR_SEND_FAILED

- 调整日志级别
  - Session lagged: warn -> debug
  - 移除 H264 NAL trace 日志
  - 移除 Frame distribution lagged trace 日志
  - 移除 absolute mouse report trace 日志

- 优化 broadcast channel 缓冲区大小 8 -> 16

- 修复条件编译
  - static_files.rs: 添加 debug_assertions 条件

src/stream/mjpeg.rs

@@ -167,7 +167,7 @@ impl MjpegStreamHandler {
 
     /// Create handler with custom drop limit
     pub fn with_drop_limit(max_drop: u64) -> Self {
-        let (frame_notify, _) = broadcast::channel(4); // Reduced from 16 for lower latency
+        let (frame_notify, _) = broadcast::channel(16); // Buffer size 16 for low latency
         Self {
             current_frame: ArcSwap::from_pointee(None),
             frame_notify,