FEAT: Undo send (delayed delivery window) #1139

Describe the feature

After a send action, hold the message in a local outbox for N seconds (configurable, default 10). The user can press u during that window to recall the message; otherwise the daemon delivers it.

Why this matters

Standard Gmail/Outlook feature; saves users from "reply-all to wrong list" disasters.

Notes

• Implementation overlaps heavily with FEAT scheduled send (#sibling). Same outbox, same daemon scanner, just a fixed delay.
• Provide a status-line countdown so the user knows the recall window is open.