Adds a 'Create new folder…' tile at the bottom of the move-to bottom sheet in the single mail view. Tapping it shows a text input dialog; on confirm, the folder is created on the server and the email is immediately moved there. - Extend MailboxRepository with createMailbox(accountId, name) - Implement it in MailboxRepositoryImpl for IMAP and JMAP, reusing existing private helpers with role made nullable - Wire up the new flow in _moveTo() on EmailDetailScreen Closes #422 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>