New spark-mail crate: IMAP inbox sync and SMTP sending (app passwords),
Microsoft Graph delta sync and sending with OAuth PKCE sign-in, MIME
parsing and Smart Inbox categories, and an engine that pushes local
changes (Done, Delete, read, pin) after the undo window and sends queued
drafts. The store now keeps accounts, messages with local and server
state, Gatekeeper sender decisions and the outbox.

The desktop app adds accounts in Settings, runs a sync worker per
account, keeps passwords and refresh tokens in the OS credential store,
shows email HTML in a script-less sandboxed frame with remote images
blocked, wakes snoozed mail on time, and sends with undo and Send Later.
The demo mailbox remains until the first account is added.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>