Feature/cc bccfeat: Add CC/BCC support in email compose

[dgcjj]

Summary

This PR adds CC and BCC support to the email compose window.

Changes

• mail-vue/src/layout/write/index.vue: Added CC/BCC input fields using el-input-tag component, consistent with the existing To field style
• mail-worker/src/service/email-service.js: Added cc and bcc parameters to send(), sendByResend(), and sendByCloudflareEmail() functions

How it works

• Resend API natively supports cc and bcc parameters
• Cloudflare Email API also supports cc and bcc
• Fields are optional and only sent when non-empty

Related Issue

Closes #561

Testing

Tested on self-hosted instance with Resend as the send provider. CC and BCC recipients receive the email correctly.