refactor(editor): delegate graph viewport actions to JSONCrackRef

Route zoomIn/zoomOut/centerView/focusFirstNode in useGraph through
the ref exposed by jsoncrack-react instead of reimplementing them
against the raw ViewPort + DOM queries. This ensures the built-in
Controls, ZoomControl buttons, and keyboard shortcuts share the
same fit logic as the package, fixing subtle divergence between
the editor's "Fit to center" and the package's internal auto-fit.