fix(GraphPanel): optimize force graph drag behavior to prevent simulation restart on click

Background:
- With many nodes, D3 force simulation tick callback updates all nodes/edges/labels every frame
- Original implementation called simulation.restart() in drag start event on mousedown
- Even after simulation converged, clicking a node to view details would restart simulation, causing lag

Solution:
- Distinguish between "click" and "drag" using 3px movement threshold
- On drag start: only record initial position, do not restart simulation
- On drag event: detect movement exceeding threshold before marking as actual drag and restarting simulation
- Pure click operations no longer trigger simulation restart, keeping graph static

Bug fix:
- Fixed issue where nodes became undraggable after initial optimization
- Cause: incorrectly used if(!event.active) check in drag event
- event.active equals 1 during drag event, causing restart() to never execute
- Removed that condition, using custom _isDragging flag to control logic instead

frontend/src/components/GraphPanel.vue

@@ -661,18 +661,38 @@ const renderGraph = () => {
     .style('cursor', 'pointer')
     .call(d3.drag()
       .on('start', (event, d) => {
-        if (!event.active) simulation.alphaTarget(0.3).restart()
+        // 只记录位置，不重启仿真（区分点击和拖拽）
         d.fx = d.x
         d.fy = d.y
+        d._dragStartX = event.x
+        d._dragStartY = event.y
+        d._isDragging = false
       })
       .on('drag', (event, d) => {
-        d.fx = event.x
-        d.fy = event.y
+        // 检测是否真正开始拖拽（移动超过阈值）
+        const dx = event.x - d._dragStartX
+        const dy = event.y - d._dragStartY
+        const distance = Math.sqrt(dx * dx + dy * dy)
+        
+        if (!d._isDragging && distance > 3) {
+          // 首次检测到真正拖拽，才重启仿真
+          d._isDragging = true
+          simulation.alphaTarget(0.3).restart()
+        }
+        
+        if (d._isDragging) {
+          d.fx = event.x
+          d.fy = event.y
+        }
       })
       .on('end', (event, d) => {
-        if (!event.active) simulation.alphaTarget(0)
+        // 只有真正拖拽过才让仿真逐渐停止
+        if (d._isDragging) {
+          simulation.alphaTarget(0)
+        }
         d.fx = null
         d.fy = null
+        d._isDragging = false
       })
     )
     .on('click', (event, d) => {