Support CC/BCC importing in MsgLoader

[lavr]

Cherry-pick the MsgLoader CC/BCC import support from #72.

The original PR also added a Bcc header to _build_root_message, which is incorrect — BCC should only be handled at the SMTP envelope level. That part should not be included.

What to do:

• Add cc and bcc handling to copy_header_to_message() in local_store.py
• Add corresponding tests
• Fix indentation from the original PR (5 spaces → 4)
• Do not add Bcc header in _build_root_message