feat(cli): configurable send-as From address (flags, TUI, validation)

- tui.ValidFromAddress: exported validator; blank passes, malformed rejects
- Fields.FromAddress: new field, round-trips through ToAccount/FieldsFromAccount
- Fields.Validate: calls ValidFromAddress before returning nil
- TUI form: from_address fieldDef between username and password
- send.go: From set via acc.SendFrom() instead of acc.Username
- admin.go account add: --from flag with pre-parse validation
- admin.go account edit: --from flag; validate before Visit, apply in Visit
- USER-MANUAL.md: --from flag added to account add flags table

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/cli/send.go

@@ -23,7 +23,7 @@ func SendCmd(d Deps, account string, to, cc, bcc []string, subject, body string,
 	}
 
 	msg := mail.OutgoingMessage{
-		From: acc.Username, To: to, Cc: cc, Bcc: bcc,
+		From: acc.SendFrom(), To: to, Cc: cc, Bcc: bcc,
 		Subject: subject, BodyText: body,
 	}
 	recipients := msg.Recipients()