fix: replace Proxmox cluster handles with configurable side points

The Proxmox node had two always-on cluster-left/cluster-right handles that
rendered independently of the new per-side connection-point config — so a
configured side stacked on top of the forced handle (e.g. 0 config still showed
1; 2 config left the forced one in the middle).

- ProxmoxGroupNode: remove the always-on cluster handles in both container and
  non-container modes; cluster links now use the normal per-side points
- add migrateClusterHandles(nodes, edges): on load, remap any edge still bound
  to cluster-left/right onto the left/right slot-0 point and set that node's
  side count to at least 1, so existing links survive (edge 'cluster' type/style
  kept). Applied on API load, standalone load, LiveView, and YAML import
- App: new cluster links are made by choosing the Cluster edge type in the edge
  modal (drop the cluster-handle auto-default)

ha-relevant: yes

frontend/src/utils/importYaml.ts

@@ -4,6 +4,7 @@ import type { NodeData, EdgeData } from '@/types'
 import type { YamlNode, YamlNodeConnection } from '@/types/yaml'
 import { generateUUID } from '@/utils/uuid'
 import { applyDagreLayout } from '@/utils/layout'
+import { migrateClusterHandles } from '@/utils/canvasSerializer'
 
 /**
  * Parse a YAML string and merge the resulting nodes/edges into the existing canvas.
@@ -170,5 +171,9 @@ export function parseYamlToCanvas(
   const mergedEdges = [...existingEdges, ...newEdges]
   const laidOut = applyDagreLayout(mergedNodes, mergedEdges)
 
-  return { nodes: laidOut, edges: mergedEdges, imported: newNodes.length }
+  // Cluster links are imported on the legacy 'cluster-left/right' handles;
+  // remap them to the per-side connection points (and give the side a point).
+  const migrated = migrateClusterHandles(laidOut, mergedEdges)
+
+  return { nodes: migrated.nodes, edges: migrated.edges, imported: newNodes.length }
 }