feat(group): add 4 side handles for edge connections

Group nodes can now be source/target of edges via snap points on
each side (top/right/bottom/left). Each side exposes a source handle
plus an invisible target overlay, matching the pattern used by
BaseNode.

frontend/src/components/canvas/__tests__/GroupNode.test.tsx

@@ -11,6 +11,8 @@ vi.mock('@xyflow/react', () => ({
   NodeResizer: ({ isVisible }: { isVisible: boolean }) => (
     <div data-testid="node-resizer" data-visible={isVisible} />
   ),
+  Handle: () => null,
+  Position: { Top: 'top', Right: 'right', Bottom: 'bottom', Left: 'left' },
   useReactFlow: () => ({}),
 }))