Closes#501
searchEmails now queries the local email_fts virtual table filtered by
mailbox_path instead of doing a live IMAP SEARCH. This makes folder-view
search work offline and ensures tapped results always open the correct
email (IDs come from the same local DB that getEmail reads from).
Reuses the existing FTS5 infrastructure (_toFtsQuery + the email_fts
content-table join) from searchEmailsGlobal, adding only the
`AND e.mailbox_path = ?` filter.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>