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/rustdesk/mod.rs

@@ -383,12 +383,6 @@ impl RustDeskService {
         self.start().await
     }
 
-    /// Get a shutdown receiver for graceful shutdown handling
-    #[allow(dead_code)]
-    pub fn shutdown_rx(&self) -> broadcast::Receiver<()> {
-        self.shutdown_tx.subscribe()
-    }
-
     /// Save keypair and UUID to config
     /// Returns the updated config if changes were made
     pub fn save_credentials(&self) -> Option<RustDeskConfig> {