fix: allow parent-to-child connections in React Flow

React Flow blocks connections involving a parentId parent/child pair by
default to prevent hierarchy cycles. Override with isValidConnection
that only rejects self-loops (source === target), allowing explicit
edges between Proxmox container nodes and their nested VMs/LXCs.

frontend/src/components/canvas/CanvasContainer.tsx

@@ -58,6 +58,7 @@ export function CanvasContainer({ onConnect: onConnectProp, onEdgeDoubleClick }:
         fitView
         colorMode="dark"
         connectionMode="loose"
+        isValidConnection={(connection) => connection.source !== connection.target}
       >
         <Background
           variant={BackgroundVariant.Dots}