Decide how sending is gated while SMTP is unverified

[Hohnik]

Part of #18

Question

What happens when the user tries to send from an account whose SMTP capability is not verified?

Options: the reply UI is hidden or disabled outright; the attempt is allowed and surfaces the server error on failure; or the send is pre-flighted against the stored state and refused with an explanation.

Note there is no send path in the codebase yet — send_message exists in sync/smtp_client.py but is called only from tests. Compose/reply lands with Reply within a conversation, so this ticket's answer becomes an acceptance criterion there rather than code here.

Also settle: does an unknown (never-checked) state gate sending the same way a failed one does?