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/stores/__tests__/canvasStore.test.ts

@@ -497,6 +497,31 @@ describe('canvasStore', () => {
     expect(useCanvasStore.getState().hasUnsavedChanges).toBe(true)
   })
 
+  it('reconnectEdge swaps source/target and normalizes handles', () => {
+    useCanvasStore.setState((s) => ({
+      edges: [...s.edges, { ...makeEdge('e1', 'n1', 'n2'), sourceHandle: 'bottom', targetHandle: 'top' }],
+    }))
+    useCanvasStore.getState().markSaved()
+    useCanvasStore.getState().reconnectEdge('e1', {
+      source: 'n1',
+      target: 'n3',
+      sourceHandle: 'bottom-2-t',
+      targetHandle: 'top-t',
+    })
+    const edge = useCanvasStore.getState().edges.find((e) => e.id === 'e1')
+    expect(edge?.target).toBe('n3')
+    expect(edge?.source).toBe('n1')
+    expect(edge?.sourceHandle).toBe('bottom-2')
+    expect(edge?.targetHandle).toBe('top')
+    expect(useCanvasStore.getState().hasUnsavedChanges).toBe(true)
+  })
+
+  it('reconnectEdge snapshots history for undo', () => {
+    useCanvasStore.setState((s) => ({ edges: [...s.edges, makeEdge('e1', 'n1', 'n2')], past: [] }))
+    useCanvasStore.getState().reconnectEdge('e1', { source: 'n1', target: 'n3', sourceHandle: null, targetHandle: null })
+    expect(useCanvasStore.getState().past.length).toBe(1)
+  })
+
   it('deleteEdge removes the edge and marks unsaved', () => {
     useCanvasStore.setState((s) => ({ edges: [...s.edges, makeEdge('e1', 'n1', 'n2'), makeEdge('e2', 'n2', 'n3')] }))
     useCanvasStore.getState().markSaved()

frontend/src/stores/canvasStore.ts

@@ -44,6 +44,7 @@ interface CanvasState {
   updateNode: (id: string, data: Partial<NodeData>) => void
   deleteNode: (id: string) => void
   updateEdge: (id: string, data: Partial<EdgeData>) => void
+  reconnectEdge: (id: string, connection: Connection) => void
   deleteEdge: (id: string) => void
   setProxmoxContainerMode: (proxmoxId: string, enabled: boolean) => void
   setNodeZIndex: (id: string, zIndex: number) => void
@@ -292,6 +293,24 @@ export const useCanvasStore = create<CanvasState>((set) => ({
       hasUnsavedChanges: true,
     })),
 
+  reconnectEdge: (id, connection) =>
+    set((state) => ({
+      edges: state.edges.map((e) =>
+        e.id === id
+          ? {
+              ...e,
+              source: connection.source ?? e.source,
+              target: connection.target ?? e.target,
+              sourceHandle: normalizeHandle(connection.sourceHandle),
+              targetHandle: normalizeHandle(connection.targetHandle),
+            }
+          : e
+      ),
+      past: [...state.past.slice(-49), { nodes: state.nodes, edges: state.edges }],
+      future: [],
+      hasUnsavedChanges: true,
+    })),
+
   deleteEdge: (id) =>
     set((state) => ({
       edges: state.edges.filter((e) => e.id !== id),