IMAP: support --after/--before date filtering

[wesm]

Context

The IMAP sync added in #193 warns and ignores the --after/--before flags for IMAP sources. Gmail sync supports these via search query syntax (after: / before:), but IMAP has no equivalent pass-through.

Proposal

Use IMAP SEARCH SINCE <date> and SEARCH BEFORE <date> criteria when --after / --before flags are provided. These operate on INTERNALDATE (date-only, no time component), which matches Gmail's behavior.

This would allow targeted date-range syncs for large IMAP mailboxes:

msgvault sync-full imaps://user@imap.mail.yahoo.com:993 --after 2024-01-01 --before 2024-12-31

Current behavior

Warning: --query/--before/--after are not supported for IMAP sources and will be ignored.

References

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