🐛 修复邮件发送功能的小问题

- 修复发件人字段处理，避免空字符串导致发送失败
- 改进SMTP测试错误消息显示，支持详细错误信息
- 更新package-lock.json锁定nodemailer版本

这些修复提升了邮件功能的稳定性和错误提示的友好性。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/package-lock.json

@@ -21,6 +21,7 @@
         "express-validator": "^7.3.0",
         "jsonwebtoken": "^9.0.2",
         "multer": "^2.0.2",
+        "nodemailer": "^6.9.14",
         "ssh2-sftp-client": "^12.0.1",
         "svg-captcha": "^1.4.0"
       },
@@ -1913,6 +1914,15 @@
         "node": ">=10"
       }
     },
+    "node_modules/nodemailer": {
+      "version": "6.10.1",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-6.10.1.tgz",
+      "integrity": "sha512-Z+iLaBGVaSjbIzQ4pX6XV41HrooLsQ10ZWPUehGmuantvzWoDVBnmsdUcOIDM1t+yPor5pDhVlDESgOMEGxhHA==",
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/nodemon": {
       "version": "3.1.10",
       "resolved": "https://registry.npmjs.org/nodemon/-/nodemon-3.1.10.tgz",