feat(ui): cross-platform screenshot hotkey and portal-based capture

Windows keeps the legacy path; Linux X11 uses a native XGrabKey hotkey (ctypes libX11) with native capture; Wayland captures through xdg-desktop-portal and degrades gracefully with explicit capability-unavailable logs when the portal/protocol is missing.

ui/views/system_tools/screen_capture.py

@@ -60,8 +60,15 @@ class ScreenCaptureOverlay(QtWidgets.QWidget):
         """开始截图：抓取屏幕全图并显示覆盖层"""
         screen = QtWidgets.QApplication.primaryScreen()
         if not screen:
+            print("[Screenshot] 无主屏幕 → 截图不可用", flush=True)
             return
         self._full_pixmap = screen.grabWindow(0)
+        # P1-04：X11 某些 compositor/环境下 grabWindow 可能拿到空图（Wayland 已改走 portal）
+        if self._full_pixmap is None or self._full_pixmap.isNull() or self._full_pixmap.width() == 0:
+            print("[Screenshot] 屏幕抓取返回空画面（当前 compositor/环境限制）→ 本次截图取消；"
+                  "聊天与其他功能不受影响", flush=True)
+            self._full_pixmap = None
+            return
         self.setGeometry(screen.geometry())
         self.show()
         self.activateWindow()