Replace 8 bare fmt.Errorf calls with output.ErrValidation across 3 files
so validation errors consistently return structured JSON (type: validation,
exit 2) matching the rest of the codebase.
Affected functions: validateExpectedFlag (sheets), validateSendTime,
validateComposeInlineAndAttachments, validateEventFlags (mail),
validateSignatureWithPlainText (mail)
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>