React Flow's built-in keyboard handling was silently no-op'd because
deleteKeyCode wasn't set (v12 defaults it to null when the prop is absent
in the strict TS types we consume). Set it to ['Delete', 'Backspace'] and
add an onNodesDelete handler that filters edges whose endpoints just
vanished — applyNodeChanges only touches nodes, so without this the graph
kept dangling edges.
Also fixes a tsc regression the same touch caught in ConnectionsPage.test
where a request.json() cast leaked 'never' back into a later access.
8c2d630af5