fix: 修复无信号状态同步与采集管线阻塞

- 使用非阻塞 V4L2 句柄并类型化采集错误
- 完善视频管线生命周期，避免复用停止中的管线
- 区分音视频事件并支持 REST/WebSocket 状态快照
- 修复信号丢失后保留最后一帧及刷新后黑屏问题
- 优化无信号提示、状态展示优先级及相关界面细节
- 降低重复 OTG 错误日志级别

src/video/capture/mod.rs

@@ -1,10 +1,51 @@
 //! Video capture implementations and capture-state helpers.
 
+use std::fmt;
+use std::io;
+
 pub(crate) mod runtime;
 pub(crate) mod status;
 
 pub const DEFAULT_CAPTURE_BUFFER_COUNT: u32 = 4;
 
+/// Expected source changes are control flow, not stringly typed I/O errors.
+#[derive(Debug)]
+pub enum CaptureReadError {
+    SourceChanged,
+    Io(io::Error),
+}
+
+impl CaptureReadError {
+    pub fn as_io_error(&self) -> Option<&io::Error> {
+        match self {
+            Self::SourceChanged => None,
+            Self::Io(error) => Some(error),
+        }
+    }
+}
+
+impl From<io::Error> for CaptureReadError {
+    fn from(error: io::Error) -> Self {
+        Self::Io(error)
+    }
+}
+
+impl fmt::Display for CaptureReadError {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::SourceChanged => formatter.write_str("capture source changed"),
+            Self::Io(error) => error.fmt(formatter),
+        }
+    }
+}
+
+impl std::error::Error for CaptureReadError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        self.as_io_error()
+            .map(|error| error as &(dyn std::error::Error + 'static))
+    }
+}
+
 #[cfg(unix)]
 mod linux;
 #[cfg(windows)]