视频导出、时间轴与动作姿势模块

- 视频导出：WebCodecs 逐帧快速导出 MP4（固定分辨率不随视口布局变形），MediaRecorder 兜底
- 时间轴：面板可拖动调高（持久化、固定上限）、关键帧轨道区内滚动、无横向滚动条
- 动作模块：走路/跑步步态曲线（对侧摆臂修复、跑步独立步频、跑姿屈膝曲线、参考校准参数）
- 姿势调节：两级折叠分组、动作偏移联动
- 删除对象时同步清理时间轴轨道

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -37,7 +37,9 @@
       "target": [
         {
           "target": "nsis",
-          "arch": ["x64"]
+          "arch": [
+            "x64"
+          ]
         }
       ],
       "icon": "public/icon.png",
@@ -57,6 +59,7 @@
     "@react-three/fiber": "^8.17.14",
     "camera-controls": "^2.9.0",
     "lucide-react": "^0.469.0",
+    "mp4-muxer": "^5.2.2",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",
     "three": "^0.184.0",