fix(zigbee): strict tree edges + auto-select imported nodes

Edges
- Z2M `links` is bidirectional and includes router mesh paths, which
  caused duplicate edges and edges entering the coordinator from the
  bottom. Walk `links` only to derive parent_id + LQI; build final
  edges strictly from the parent->child hierarchy (one edge per
  non-coordinator node). Result: parent bottom -> child top, every time.
- Tests: bidirectional pair collapses to one edge, router-mesh siblings
  dropped, coordinator never receives an edge.

Auto-select
- After import, deselect existing canvas nodes and mark only the
  freshly-imported ones as selected, so the user can drag the whole
  subtree as a group.

frontend/src/App.tsx

@@ -345,18 +345,24 @@ export default function App() {
       }
       addNode(newNode)
     })
-    // Add IoT edges between Zigbee devices
+    // Add IoT edges between Zigbee devices: parent bottom -> child top
     zigbeeEdges.forEach((ze) => {
-      const sourceId = ze.source
-      const targetId = ze.target
       onConnect({
-        source: sourceId,
-        sourceHandle: 'top',
-        target: targetId,
+        source: ze.source,
+        sourceHandle: 'bottom',
+        target: ze.target,
         targetHandle: 'top-t',
         type: 'iot',
       } as unknown as import('@xyflow/react').Connection)
     })
+    // Auto-select only the freshly imported nodes so the user can drag the
+    // whole subtree as a group.
+    const importedIds = new Set(zigbeeNodes.map((zn) => zn.id))
+    useCanvasStore.setState((state) => ({
+      nodes: state.nodes.map((n) => ({ ...n, selected: importedIds.has(n.id) })),
+      selectedNodeIds: Array.from(importedIds),
+      selectedNodeId: importedIds.size === 1 ? Array.from(importedIds)[0] : null,
+    }))
     markUnsaved()
   }, [addNode, onConnect, snapshotHistory, markUnsaved])