fix: edges appear immediately + sync virtual link with LXC parent_id

- Normalize stub handle IDs (top-t/bottom-t → top/bottom) in canvasStore.onConnect
  so React Flow can locate the handle and render edges without save+reload
- Also normalize on save in App.tsx as a safety net for persisted handle IDs
- Auto-create virtual edge (LXC top → Proxmox bottom) when parent_id is set
  via the node edit modal, and remove it when parent_id is cleared/changed
- Auto-set LXC/VM parent_id when a virtual edge is drawn to/from a Proxmox node

frontend/src/stores/canvasStore.ts

@@ -56,9 +56,15 @@ export const useCanvasStore = create<CanvasState>((set) => ({
     set((state) => {
       const extra = connection as Connection & Partial<EdgeData>
       const edgeType = extra.type ?? 'ethernet'
+      // Normalize invisible stub handle IDs so React Flow can locate the handle
+      // and render the edge immediately (top-t / bottom-t are opacity:0 helpers).
+      const normalizeHandle = (h: string | null | undefined) =>
+        h === 'top-t' ? 'top' : h === 'bottom-t' ? 'bottom' : (h ?? null)
       return {
         edges: addEdge({
           ...connection,
+          sourceHandle: normalizeHandle(extra.sourceHandle),
+          targetHandle: normalizeHandle(extra.targetHandle),
           type: edgeType,
           data: { type: edgeType, label: extra.label, vlan_id: extra.vlan_id, custom_color: extra.custom_color, path_style: extra.path_style },
         }, state.edges),