fix: 解决邮件发送错误，但是前端显示已发送的问题

backend/src/main/java/com/openisle/exception/EmailSendException.java

@@ -0,0 +1,15 @@
+package com.openisle.exception;
+
+/**
+ * Thrown when email sending fails so callers can surface a clear error upstream.
+ */
+public class EmailSendException extends RuntimeException {
+
+  public EmailSendException(String message) {
+    super(message);
+  }
+
+  public EmailSendException(String message, Throwable cause) {
+    super(message, cause);
+  }
+}