Add bulk edit for row properties

[priethor]

Goal

Editing one property across many rows is a routine task ("set Status to Done on these five"). Today you do it row by row. Add a bulk-edit option from the row selection.

Scope

• From a multi-row selection, a Bulk edit action opens a modal.
• The modal lets you pick one editable field and either set a value or clear it.
• Editable: any field that holds a stored value, including relations.
• For multi-value fields (multiselect, relation), the new value replaces the existing one on each row.
• Result feedback: a summary of how many rows updated and how many failed.

Out of scope

• Title, formula, and rollup. Title is each row's identity; formula and rollup are computed, no value to set.
• Conditional bulk edits, e.g. "set Status only if currently In Progress".

Acceptance criteria

• From a multi-row selection, Bulk edit opens a modal.
• Picking a field and a value updates all selected rows; the result shows success and failure counts.
• The clear-value toggle sets the field to empty on every selected row.
• Multi-value fields replace, not merge.