预览拖动阈值抽成公共常量模块

- 新增 previewDragThreshold.ts，导出 DRAG_THRESHOLD_SCREEN_PX 与屏幕坐标阈值判定 passedDragThresholdScreen
- useNodeTransformInteraction 删除本地阈值常量，改为复用该模块，左键拖动/缩放行为不变

验证：npx vitest run useNodeTransformInteraction.test.tsx previewWorkspaceZoom.test.tsx（18 passed）

apps/ai-game-creator-shell/src/view/ui-editor/components/preview/previewDragThreshold.ts

@@ -0,0 +1,21 @@
+/** 屏幕像素坐标点。 */
+export type ScreenPoint = {
+  x: number;
+  y: number;
+};
+
+/**
+ * 指针拖动阈值（屏幕像素）。左键拖动/缩放与右键拖拽平移共用同一判据，
+ * 保证同一张画布上"多小的移动算点击"只有一个答案。
+ */
+export const DRAG_THRESHOLD_SCREEN_PX = 2;
+
+export function passedDragThresholdScreen(
+  start: ScreenPoint,
+  current: ScreenPoint,
+): boolean {
+  return (
+    Math.hypot(current.x - start.x, current.y - start.y) >=
+    DRAG_THRESHOLD_SCREEN_PX
+  );
+}

apps/ai-game-creator-shell/src/view/ui-editor/components/preview/useNodeTransformInteraction.ts

@@ -21,6 +21,7 @@ import {
 import type { Node as UiNode } from '../../../../features/ui-editor/types/Node';
 import type { UITree } from '../../../../features/ui-editor/types/UITree';
 import type { UiEditorCanvasProjection } from '../../useUiEditorPage';
+import { passedDragThresholdScreen } from './previewDragThreshold';
 
 type ViewportScale = { scale: number };
 type GestureBase = {
@@ -47,17 +48,13 @@ type ActiveGesture =
       ratioAxis: ResizeAxis | null;
     });
 
-const DRAG_THRESHOLD_SCREEN_PX = 2;
-
 function passedDragThreshold(
   gesture: GestureBase,
   event: ReactPointerEvent<HTMLDivElement>,
 ) {
-  return (
-    Math.hypot(
-      event.clientX - gesture.startClientX,
-      event.clientY - gesture.startClientY,
-    ) >= DRAG_THRESHOLD_SCREEN_PX
+  return passedDragThresholdScreen(
+    { x: gesture.startClientX, y: gesture.startClientY },
+    { x: event.clientX, y: event.clientY },
   );
 }