Add header text to multiple selection; improve header consistency between modes

Editor assessment

The scope `datetime` is right, but the subject tries to carry two changes at once — adding header text for multiple selection and improving header consistency between modes — joined by a semicolon, and it balloons past 90 characters, guaranteeing truncation in GitHub's log view. Convention and this project's own history favor splitting into two commits or foregrounding the primary change. There is no body to absorb the second change either; the co-author trailer is present but no rationale for the consistency rework. `feat` itself is correctly chosen for the new header text.