feat(canvas): edge endpoint reconnect + proxmox container snap points

- Selected edge shows interactive dots at both endpoints; drag onto any
  handle (different snap point or different node) to reconnect. Edge
  follows the cursor live during drag; drop on empty space leaves edge
  unchanged.
- Endpoint dots render in EdgeLabelRenderer above the node layer and walk
  the elementsFromPoint stack so the underlying handle wins over the
  dragged dot itself.
- ProxmoxGroupNode in container mode now renders bottom_handles snap
  points (was always single 'bottom' handle), matching BaseNode.
- New canvasStore.reconnectEdge action: swaps source/target/handles via
  normalizeHandle, snapshots history, marks dirty.

frontend/src/components/canvas/nodes/__tests__/ProxmoxGroupNode.test.tsx

@@ -102,6 +102,18 @@ describe('ProxmoxGroupNode', () => {
     expect(container.querySelector('.rounded-xl.border-2')).not.toBeNull()
   })
 
+  it('container mode renders bottom_handles snap points', () => {
+    const { container } = renderNode({ bottom_handles: 4 })
+    const sourceHandles = container.querySelectorAll('.react-flow__handle-bottom.source')
+    expect(sourceHandles.length).toBe(4)
+  })
+
+  it('container mode default has single bottom handle', () => {
+    const { container } = renderNode({})
+    const sourceHandles = container.querySelectorAll('.react-flow__handle-bottom.source')
+    expect(sourceHandles.length).toBe(1)
+  })
+
   it('renders cluster handles in both modes', () => {
     const { container: groupC } = renderNode({})
     expect(groupC.querySelectorAll('[title="Same cluster"]').length).toBeGreaterThanOrEqual(2)