Use default channel when replying to EML attachments

Problem
When replying to an EML file opened in Front, the reply channel is selected from a fixed list (private channels first, then shared channels alphabetically), which often doesn't match the user's preferred default channel. This causes frustration.

Request
When replying to an EML attachment, prioritize the user's default shared channel in the channel selection order, before falling back to other available channels.

Context
Proposed channel selection order: Available private channel → Default shared channel (if set) → First available shared channel (alphabetically).