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/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),