Add gravitational forces to node simulation in GraphPanel.vue

- Introduced x and y forces to attract independent nodes towards the center, enhancing the overall layout and clustering of nodes in the graph visualization.
- Improved the simulation dynamics for a more cohesive visual representation of the graph.

frontend/src/components/GraphPanel.vue

@@ -440,6 +440,9 @@ const renderGraph = () => {
     .force('charge', d3.forceManyBody().strength(-400))
     .force('center', d3.forceCenter(width / 2, height / 2))
     .force('collide', d3.forceCollide(50))
+    // 添加向中心的引力，让独立的节点群聚集到中心区域
+    .force('x', d3.forceX(width / 2).strength(0.04))
+    .force('y', d3.forceY(height / 2).strength(0.04))
   
   currentSimulation = simulation