Add bulk row selection and bulk delete

[priethor]

Goal

With the per-row actions menu in place (#107), the next step is selecting many rows and deleting them at once.

Scope

• Selection in the DataView block: shift-click for ranges, ctrl/cmd-click for non-contiguous, and the selection survives pagination.
• Bulk delete from the same per-row Delete action, with the same confirmation step.

Out of scope

• Bulk edit of property values.
• Bulk move across collections.

Acceptance criteria

• Shift-click and ctrl-click select rows on layouts that support selection.
• When rows are selected, a bulk action lets you delete them at once.
• Confirmed bulk delete removes the selected rows and clears the selection.
• Selecting on page 1 and navigating to page 2 keeps the selection.