feat: 优化测量功能架构与引擎组件

- 重构测量激活逻辑，在 Engine 组件中添加统一的 activateMeasure(mode) 方法
- 简化 MeasureDialogManager，移除冗余的 handleMeasureTypeChange 方法
- 添加 EngineManager.activateMeasure 转发方法
- 修复 loadModel 错误，正确调用 Engine 组件方法
- 为 Engine 组件设置固定背景渐变色
- MeasurePanel 初始化时触发 onModeChange 回调
- 添加 MeasureMode 共享类型定义

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

demo/package.json

@@ -7,7 +7,9 @@
     "dev": "vite",
     "build": "vite build",
     "preview": "vite preview",
-    "copy-sdk": "mkdir -p lib && cp ../dist/bim-engine-sdk.umd.js lib/ && cp ../dist/bim-engine-sdk.umd.js.map lib/ 2>/dev/null || true"
+    "copy-sdk": "mkdir -p lib && cp ../dist/bim-engine-sdk.umd.js lib/ && cp ../dist/bim-engine-sdk.umd.js.map lib/ 2>/dev/null || true",
+    "copy-draco": "mkdir -p static/js/draco && cp ../src/engine_base/draco/*.js ../src/engine_base/draco/*.wasm static/js/draco/ 2>/dev/null || true",
+    "copy-all": "npm run copy-sdk && npm run copy-draco"
   },
   "devDependencies": {
     "vite": "^7.2.6"