Adds a minimal ManageSieve client so the existing "Email filters" UI
works for IMAP accounts, not just JMAP. SieveRepository becomes a
dispatcher that routes to JMAP or ManageSieve based on account.type.
Account model + DB schema v15 grow manageSieveHost/Port/Ssl fields
(default 4190 / TLS, host falls back to imapHost when blank). The Add
and Edit account screens expose them inside a collapsed ExpansionTile
to keep the form short for users who accept defaults.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
