selectRangeEmails was only wired to shift-clicking the row, but the
checkbox handler called stopPropagation and a plain toggle — so shift-
clicking checkboxes (the obvious affordance in selection mode) selected
single messages instead of the range. Make all three checkbox handlers
(email-list-item, thread single-email, thread header) shift-aware:
shift -> selectRangeEmails, otherwise toggle. Adds a regression test.