Board: multi-select + bulk actions

[sjcotto]

List and Table support multi-select + the shared BulkBar (set status/priority/assignee, delete). Board does not.

Scope

• Add per-card selection (checkbox on hover) to components/views/board-view.tsx, reusing the selected Set pattern and BulkBar.
• Make sure selection coexists with the existing dnd-kit drag (don't start a drag when toggling a checkbox).

Acceptance

• Select cards across columns and apply a bulk action.