Feat: window capture via PrintWindow with server-side HWND routing by clientID

client/ScreenManager.h

@@ -64,6 +64,7 @@ public:
     BOOL  m_bIsWorking;
     BOOL  m_bIsBlockInput;
     RECT  m_ROI = {0};
+    HWND  m_hTargetWnd = NULL;  // 窗口捕获目标（NULL=全屏）
     BOOL  m_SwitchScreen = TRUE;
     BOOL SendClientClipboard(BOOL fast);
     VOID UpdateClientClipboard(char *szBuffer, ULONG ulLength);