Starting a new message while viewing a specific mailbox/account now defaults
the From identity to that mailbox instead of the global primary identity, so
composing from info@ sends as info@. Mirrors the existing reply-time identity
match and rides the same autoSelectReplyIdentity setting; reply/replyAll/forward
keep resolving from the original recipients. Matches exact then +tag-stripped.
Extracts findComposeIdentityId into lib/reply-identity.ts with unit tests.