新增 EQ撤销功能，用户调节的参数不满意可以点击回退到上一次，撤销栈最多存放 30 步

client/src/pages/eq/components/FreqChart.tsx

@@ -40,6 +40,8 @@ export function FreqChart({
   onApplyB,
   onSaveB,
   onBandDrag,
+  onBandDragStart,
+  onBandDragEnd,
   onBandSelect,
   eqUi,
 }: {
@@ -53,6 +55,8 @@ export function FreqChart({
   onApplyB: () => void;
   onSaveB: () => void;
   onBandDrag: (idx: number, patch: Partial<{ freq: number; gain: number }>) => void;
+  onBandDragStart?: () => void;
+  onBandDragEnd?: () => void;
   onBandSelect: (idx: number) => void;
   eqUi: PeqEqUi;
 }) {
@@ -118,6 +122,7 @@ export function FreqChart({
     e.stopPropagation();
     draggingBandRef.current = idx;
     setIsBandDragging(true);
+    onBandDragStart?.();
     onBandSelect(idx);
     updateBandFromPointer(idx, e.clientX, e.clientY);
   };
@@ -130,8 +135,10 @@ export function FreqChart({
   };
 
   const stopDragging = () => {
+    if (draggingBandRef.current === null) return;
     draggingBandRef.current = null;
     setIsBandDragging(false);
+    onBandDragEnd?.();
   };
 
   const curveData = useMemo(