[feat] add Windows privacy screen and prefer DXGI capture

apps/desktop/src/privacy/privacy_backend.h

@@ -0,0 +1,64 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-09-10
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _PRIVACY_BACKEND_H_
+#define _PRIVACY_BACKEND_H_
+
+#include <cstdint>
+#include <memory>
+#include <string>
+
+namespace crossdesk {
+
+struct PrivacyScreenText {
+  std::string verification = "Privacy screen verification";
+  std::string unlock_hint = "To turn off the privacy screen, press";
+};
+
+struct PrivacyCapabilities {
+  bool overlay = false;
+  bool block_input = false;
+  std::string reason;
+};
+
+struct PrivacyHealth {
+  std::string failure;
+  bool emergency_exit = false;
+};
+
+// Lifecycle, message pumping and native resource operations run on the
+// controller's message thread. Const metadata queries are serialized with
+// those operations by the controller mutex and must not call native UI APIs.
+// No platform backend is installed on macOS/Linux yet.
+class PrivacyBackend {
+ public:
+  virtual ~PrivacyBackend() = default;
+  virtual PrivacyCapabilities Query() = 0;
+  virtual bool Enable(bool block_input, const PrivacyScreenText& text,
+                      std::string& error) = 0;
+  virtual void Recover() = 0;
+  virtual bool IsRecovered() const = 0;
+  virtual PrivacyHealth Poll(bool force_check = false) = 0;
+  // Wait on both native window messages and controller commands.
+  // Wake is the only backend operation callable from another thread.
+  virtual void WaitForEvents(unsigned timeout_ms) = 0;
+  virtual void Wake() = 0;
+  virtual bool PaintChallenge(uint32_t value, const std::string& title,
+                              std::string& error) = 0;
+  // Remove only the temporary capture probes, keeping all privacy covers up.
+  // The controller drains queued probe frames before admitting clean video.
+  virtual bool ClearChallenges(std::string& error) = 0;
+  virtual bool HasMonitor(int left, int top, int width, int height) const = 0;
+};
+
+#ifdef _WIN32
+std::unique_ptr<PrivacyBackend> CreateWindowsPrivacyBackend();
+bool IsWindowsPrivacyDesktopAvailable();
+#endif
+
+}  // namespace crossdesk
+
+#endif  // _PRIVACY_BACKEND_H_