[feat] add Linux X11 privacy screen and automatic recovery

apps/desktop/src/platform/linux/privacy/privacy_guard.h

@@ -0,0 +1,42 @@
+/*
+ * @Author: DI JUNKUN
+ * @Date: 2026-09-13
+ * Copyright (c) 2026 by DI JUNKUN, All Rights Reserved.
+ */
+
+#ifndef _PRIVACY_GUARD_H_
+#define _PRIVACY_GUARD_H_
+
+#include <cstdint>
+
+namespace crossdesk {
+
+inline constexpr char kLinuxPrivacyGuardArgument[] = "--linux-privacy-guard";
+inline constexpr int kLinuxPrivacyGuardSocket = 3;
+
+// Private protocol over an inherited SOCK_SEQPACKET socket. No listening
+// socket, shell, elevated privileges or separately installed helper is needed.
+enum class LinuxPrivacyCommand : uint8_t {
+  query,
+  enable,
+  enable_blocking,
+  recover,
+  heartbeat,
+};
+
+struct LinuxPrivacyReply {
+  uint64_t generation = 0;
+  bool supported = false;
+  bool input_supported = false;
+  bool active = false;
+  bool recovered = true;
+  bool emergency = false;
+  char capability_reason[256]{};
+  char failure[256]{};
+};
+
+int RunLinuxPrivacyGuard();
+
+}  // namespace crossdesk
+
+#endif