增强 SSH/SFTP 稳定性并完善安全校验与前端交互

backend/src/main/java/com/sshmanager/exception/AccessDeniedException.java

@@ -0,0 +1,7 @@
+package com.sshmanager.exception;
+
+public class AccessDeniedException extends SshManagerException {
+    public AccessDeniedException(String message) {
+        super(403, "ACCESS_DENIED", message);
+    }
+}