Add send log and desktop notification for send_at failures

[NewGraphEnvironment]

Problem

send_at background processes fail silently. When the machine sleeps past the grace window, stop() fires inside callr::r_bg() but nobody is listening. The user has no way to know the email wasn't sent.

Proposed Solution

1. Log to file — write success/failure to ~/.mc/send_log.txt with timestamp, recipient, subject, and outcome
2. macOS desktop notification — call osascript -e 'display notification ...' on both success and failure so the user sees a pop-up

Both should fire inside the background process after the send attempt or grace window check.

Relates to NewGraphEnvironment/sred-2025-2026#4