ConnectProvider computed HMAC over bare state, but Callback always
verifies HMAC(state+":"+intent). This caused the account-linking
flow to always fail with invalid_state.
Extracts Mailer interface, EmailData, and Button to pkg/email/types.go
so the cloud repo can use them via ServerContext. internal/email re-exports
the types as aliases so existing callers are unchanged. Also fixes
pre-existing lint errors (unchecked rollback and deadline calls).
