feat: 截图线程 + 编码线程。

每个显示器分别使用一个截图线程和一个编码（png+base64）线程。避免程序不明原因崩溃。

src-tauri/src/screenshot.rs

@@ -0,0 +1,42 @@
+use std::sync::Arc;
+
+use serde::Serialize;
+use tauri::async_runtime::RwLock;
+
+#[derive(Debug, Clone)]
+pub struct Screenshot {
+    pub display_id: u32,
+    pub height: u32,
+    pub width: u32,
+    pub bytes_per_row: usize,
+    pub bytes: Arc<RwLock<Vec<u8>>>,
+    pub scale_factor: f32,
+}
+
+impl Screenshot {
+    pub fn new(
+        display_id: u32,
+        height: u32,
+        width: u32,
+        bytes_per_row: usize,
+        bytes: Vec<u8>,
+        scale_factor: f32,
+    ) -> Self {
+        Self {
+            display_id,
+            height,
+            width,
+            bytes_per_row,
+            bytes: Arc::new(RwLock::new(bytes)),
+            scale_factor,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct ScreenshotPayload {
+    pub display_id: u32,
+    pub height: u32,
+    pub width: u32,
+    pub base64_image: String,
+}