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/__tests__/importYaml.test.ts

@@ -105,7 +105,7 @@ describe('parseYamlToCanvas', () => {
     expect(edges[0].data?.type).toBe('fibre')
   })
 
-  it('cluster edges have cluster-right→cluster-left handles', () => {
+  it('cluster edges are remapped onto right→left connection points (#243)', () => {
     const yaml = `
 - nodeType: proxmox
   label: "PVE1"
@@ -115,10 +115,15 @@ describe('parseYamlToCanvas', () => {
 - nodeType: proxmox
   label: "PVE2"
 `
-    const { edges } = parseYamlToCanvas(yaml, empty, emptyEdges)
+    const { nodes, edges } = parseYamlToCanvas(yaml, empty, emptyEdges)
     expect(edges).toHaveLength(1)
-    expect(edges[0].sourceHandle).toBe('cluster-right')
-    expect(edges[0].targetHandle).toBe('cluster-left')
+    expect(edges[0].sourceHandle).toBe('right')
+    expect(edges[0].targetHandle).toBe('left')
+    // Connected sides get a connection point so the link is anchored.
+    const src = nodes.find((n) => n.id === edges[0].source)!
+    const tgt = nodes.find((n) => n.id === edges[0].target)!
+    expect(src.data.right_handles).toBe(1)
+    expect(tgt.data.left_handles).toBe(1)
   })
 
   it('parent relationship sets parentId and creates an edge', () => {