feat(mail): 申请邮箱统一转为小写

src/views/mail/Mailbox.vue

@@ -197,6 +197,8 @@ async function submitApply(): Promise<void> {
     ElMessage.warning('两次输入的密码不一致')
     return
   }
+  // 统一小写：邮箱本地部分大小写不敏感，与后端/Stalwart 保持一致
+  f.MailboxName = f.MailboxName.toLowerCase()
   try {
     const result = await mailApi.apply({ MailboxName: f.MailboxName, Password: f.Password })
     ElMessage.success(`邮箱 ${result.MailboxName} 申请成功`)