feat(frontend): delete the selected rows with Delete and the cell menu (closes #644)

[danielnuld]

Closes #644.

• Delete key (edit mode): marks the marked rows, or the selected cell's row, for deletion. Pressing it on an all-deleted selection restores them; a mixed selection becomes all-deleted (toggleDeleteRows).
• Context menu: "Delete row / Delete N rows" (or "Undo delete"), using the same targets as the other row actions. Shown disabled with the reason when the table has no primary key or the grid isn't in edit mode, like "Set NULL".
• Deletes are only staged: nothing runs before the existing SQL preview, which serves as the confirmation.
• A row's mark checkbox no longer swallows the grid's row keys. In edit mode the cells are inputs, so the checkbox is how rows get marked, and without this Delete never reached the grid.

Tests: editSession.test.ts (toggleDeleteRows) and ResultGrid.test.tsx (Delete in edit mode from the checkbox; ignored outside edit mode). Typecheck and tests pass. Not yet tried in the running app.