IMAP: classify standard folders as system labels

[wesm]

Context

The IMAP ListLabels implementation in #193 only marks INBOX as label_type = "system" — all other mailboxes are "user". Standard IMAP folders like Sent, Drafts, Trash, and Junk should also be classified as system labels for consistency with Gmail label handling.

Proposal

Use RFC 6154 special-use attributes (\Sent, \Drafts, \Trash, \Junk, \All, \Archive) to determine label type. Fall back to common folder name matching (e.g. "Sent", "Draft", "Drafts", "Trash", "Bulk Mail") for servers that don't advertise special-use attributes.

Current behavior

client.go:543-548 — only INBOX gets label_type = "system":

labelType := "user"
if item.Mailbox == "INBOX" {
    labelType = "system"
}

References

• Yahoo Mail PRD: Design for Yahoo Mail integration #93
• IMAP support: Add IMAP account support via go-imap/v2 #193