Implement multi-recipient autocompletion and fix attachment forwarding

[kurokirasama]

This PR introduces two significant improvements to the cmdg terminal email client:

🚀 Features & Bug Fixes

1. Multi-recipient Autocompletion:
   • Upgraded the To> prompt to a new MultiSelection dialog.
   • Supports entering multiple recipients separated by commas (,) or semicolons (;).
   • Integrated Tab and Right-Arrow for quick completion and continuation.
   • Added basic email format validation before submission.
2. Fix Attachment Forwarding Bug:
   • Resolved an issue where original attachments were lost during forwarding.
   • Implemented automatic extraction and downloading of attachments from original messages.
   • Updated function signatures to ensure consistent attachment propagation.
3. Stability & Testing:
   • Fixed a nil map panic in the NewFake() testing helper.
   • Added comprehensive mock GMail API tests for forwarding and MIME generation.
   • Ensured strict gofmt compliance across all modified files.

📖 Documentation

• Updated the README.md with instructions on using the new multi-recipient autocompletion shortcuts.

[ThomasHabets]

Thank you!

One of the commits in this PR fixes the forwarding issue.

[ThomasHabets]

This broke using me to send yourself an email, but I'll fix that. I'll also remove the requirement for an email address to have a period. foo@se is technically a valid email.