画布导出的目录名单一来源

- src/components/image-editor/useImageCanvasAssetExportWorkflow.ts：images / media / 3d_models 三个目录名提成 EXPORT_FOLDER 常量，建目录、挑目录与按层分流共用同一份字面量（写错目录名不会编译报错，只会在 zip 里悄悄换目录）

src/components/image-editor/useImageCanvasAssetExportWorkflow.ts

@@ -50,16 +50,29 @@ function hasReadableLayerExportSource(layer: CanvasLayer) {
   return hasReadableAssetSource(layer.src, layer.objectKey);
 }
 
-type CanvasAssetExportFolderName = 'images' | 'media' | '3d_models';
+/**
+ * 导出目录名。名字在「挑目录」「建目录」「按层分流」三处都要用，
+ * 只留这一份字面量：写错目录名不会编译报错，只会在 zip 里悄悄换目录。
+ */
+const EXPORT_FOLDER = {
+  images: 'images',
+  media: 'media',
+  models: '3d_models',
+} as const;
+
+type CanvasAssetExportFolderName =
+  (typeof EXPORT_FOLDER)[keyof typeof EXPORT_FOLDER];
 
 /** 导出目录按媒体族分：图片 `images/`、音视频 `media/`、3D 模型 `3d_models/`。 */
 function resolveLayerExportFolderName(
   layer: CanvasLayer,
 ): CanvasAssetExportFolderName {
   if (layer.assetKind === 'model3d') {
-    return '3d_models';
+    return EXPORT_FOLDER.models;
   }
-  return (layer.mediaType ?? 'image') === 'image' ? 'images' : 'media';
+  return (layer.mediaType ?? 'image') === 'image'
+    ? EXPORT_FOLDER.images
+    : EXPORT_FOLDER.media;
 }
 
 /** zip 目标目录：三类各一个，模型不进图片目录。 */
@@ -67,10 +80,10 @@ function resolveExportTargetFolder(
   folderName: CanvasAssetExportFolderName,
   folders: { imagesFolder: JSZip; mediaFolder: JSZip; modelsFolder: JSZip },
 ) {
-  if (folderName === '3d_models') {
+  if (folderName === EXPORT_FOLDER.models) {
     return folders.modelsFolder;
   }
-  if (folderName === 'media') {
+  if (folderName === EXPORT_FOLDER.media) {
     return folders.mediaFolder;
   }
   return folders.imagesFolder;
@@ -496,11 +509,14 @@ export function useImageCanvasAssetExportWorkflow({
         const rootFolderName = `${projectName}-${archiveLabel}`;
         const zip = new JSZip();
         const rootFolder = zip.folder(rootFolderName) ?? zip;
-        const imagesFolder = rootFolder.folder('images') ?? rootFolder;
-        const mediaFolder = rootFolder.folder('media') ?? rootFolder;
+        const imagesFolder =
+          rootFolder.folder(EXPORT_FOLDER.images) ?? rootFolder;
+        const mediaFolder =
+          rootFolder.folder(EXPORT_FOLDER.media) ?? rootFolder;
         // 3D 资源导出的是模型本体（.glb/.gltf/.fbx），混进 images/ 会让人以为
         // zip 里是坏图片，所以单独一个目录。
-        const modelsFolder = rootFolder.folder('3d_models') ?? rootFolder;
+        const modelsFolder =
+          rootFolder.folder(EXPORT_FOLDER.models) ?? rootFolder;
         const imageByKey = new Map<string, CanvasAssetExportImage>();
         const usedFileNames = new Map<string, number>();
         const plannedKeys = new Set<string>();