feat: window screenshot (PrintWindow), name-based window resolution

crates/common/src/protocol.rs

@@ -15,6 +15,8 @@ pub struct WindowInfo {
 pub enum ServerMessage {
     /// Request a screenshot from the client
     ScreenshotRequest { request_id: Uuid },
+    /// Capture a specific window by its HWND (works even if behind other windows)
+    WindowScreenshotRequest { request_id: Uuid, window_id: u64 },
     /// Show a MessageBox on the client asking the user to do something.
     /// Blocks until the user clicks OK — use this when you need the user
     /// to perform a manual action before continuing.