[feat] add macOS privacy screen and keep remote sessions running

apps/desktop/src/privacy/privacy_backend.h

@@ -7,14 +7,12 @@
 #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";
 };
 
@@ -27,12 +25,13 @@ struct PrivacyCapabilities {
 struct PrivacyHealth {
   std::string failure;
   bool emergency_exit = false;
+  bool ready = true;
 };
 
 // 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.
+// AppKit work is queued to the main thread without waiting under this mutex.
 class PrivacyBackend {
  public:
   virtual ~PrivacyBackend() = default;
@@ -41,23 +40,23 @@ class PrivacyBackend {
                       std::string& error) = 0;
   virtual void Recover() = 0;
   virtual bool IsRecovered() const = 0;
-  virtual PrivacyHealth Poll(bool force_check = false) = 0;
+  virtual bool RecoveryPending() const { return false; }
+  virtual PrivacyHealth Poll() = 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
+#ifdef __APPLE__
+std::unique_ptr<PrivacyBackend> CreateMacPrivacyBackend();
+// Flush queued AppKit cleanup after joining the controller at app shutdown.
+void FlushMacPrivacyTasks();
+#endif
 
 }  // namespace crossdesk