fix(email): Apply labels to drafts

[evanhutnik]

Summary

• thread_labels.rs: Draft messages (no provider_id) are now separated from provider messages. Drafts are added directly to successful_ids for DB label updates instead of being skipped entirely. Gmail API calls are only made for provider messages.
• archived.rs: Drafts get their INBOX label added/removed in the DB when archiving/unarchiving, but are excluded from the Gmail API batch call since they don't exist in Gmail.
• seen.rs: Same pattern — drafts get their UNREAD label removed and read status updated in the DB, but aren't sent to the Gmail API.

Previously, label operations (TRASH, INBOX, UNREAD, etc.) completely skipped draft messages, which meant trashed threads with drafts would still appear in importance-filtered inbox queries because the draft messages never received the TRASH label.