Add multi-row deletion via context menu

[thomaswasle]

feat(data-grid): multi-row deletion via context menu

Users could previously only delete one row at a time from the context menu, even when multiple
rows were selected. This change makes the context menu deletion respect the current selection.

What changed

Multi-row delete via context menu

• Right-click on any row that is part of a multi-selection → context menu now shows "Delete X
  rows" and marks all selected rows for deletion at once
• Right-clicking a row that is not part of the current selection still deletes only that single
  row — no change to existing behaviour
• Works for both regular rows and unsaved insertion rows (new rows are discarded, existing rows
  are marked for deletion as before and still require an explicit save)

Bug fix: stale state overwrites

• The previous per-row callback (onMarkForDeletion) read pendingDeletions from a ref snapshot on
  every call. In a loop this meant each call saw the same stale object and only the last pk value
  survived. Fixed by collecting all pk values first and applying them in a single updateTab call
  via the new onMarkMultipleForDeletion prop.

i18n

• Added deleteRows key with {{count}} interpolation to all 6 supported locales: en, de, es, fr,
  it, zh