Make an invalid account transport mix unrepresentable: an account now
carries exactly one transport instead of a flat bag of optional
imap/jmap/dav fields.
- proto: AccountConfig gains `oneof transport { ImapSmtpTransport |
JmapTransport | DavTransport }`; the flat fields (protocol,
server_host/port, use_tls, imap/smtp tls modes, smtp_host/port,
collection_url) are removed and their field numbers reserved. Golden
descriptor regenerated; contract_stability passes.
- core: AccountConfig carries a `Transport` enum; validate() checks the
active variant and preserves the cleartext-to-remote-host refusal per
transport.
- store: physical accounts columns stay flat; the existing `protocol`
column is the transport-kind discriminator. Each variant round-trips.
- daemon: grpc maps core Transport <-> proto oneof both ways; an unset
transport returns typed VALIDATION_FAILED, never a panic.
- cli: `account add --protocol` gates transport flags; list/show render
the active transport; --json stays coherent.
Refs #278
James Maes