fix: 图片导入在裁剪前不再释放解码后的原图

拆分 use-question-import 时，「离开页面释放原图」的 effect 依赖了每次渲染都会
变化的 imageFlow 对象，选好照片后的重新渲染就会释放原图，确认裁剪时 drawImage
报 "The image source is detached"，图片导入无法创建识别任务。

- 释放逻辑移回持有原图引用的 useImageImportFlow，只在卸载时执行
- 新增单测：选好照片后重新渲染不释放原图，离开页面时释放一次

Author: margrop

src/__tests__/unit/use-question-import.test.tsx

@@ -127,6 +127,24 @@ describe('题目导入流程', () => {
         expect(mocks.post).not.toHaveBeenCalledWith('/api/analyze', expect.anything(), expect.anything());
     });
 
+    it('选好照片后页面重新渲染不会释放解码后的原图，只在离开页面时释放', async () => {
+        mocks.createImportTask.mockResolvedValueOnce(task());
+        mocks.fetchImportTask.mockResolvedValue(task());
+        await render();
+        await act(async () => { await flow.selectImage(new File(['x'], 'photo.heic', { type: 'image/heic' })); });
+        // 裁剪器打开等状态变化会触发多次重新渲染，此时原图必须保留，裁剪还要从原图绘制
+        await render();
+        expect(pipeline.dispose).not.toHaveBeenCalled();
+
+        await act(async () => { await flow.cropRegions(FULL, false); });
+        expect(pipeline.renderRegion).toHaveBeenCalledWith(prepared, FULL[0]);
+        expect(pipeline.dispose).not.toHaveBeenCalled();
+
+        await act(async () => root.unmount());
+        expect(pipeline.dispose).toHaveBeenCalledTimes(1);
+        root = createRoot(container);
+    });
+
     it('识别中时页面显示任务状态卡，可继续操作', async () => {
         mocks.createImportTask.mockResolvedValueOnce(task());
         mocks.fetchImportTask.mockResolvedValue(task());

src/lib/hooks/use-image-import-flow.ts

@@ -1,6 +1,6 @@
 "use client";
 
-import { useRef, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { useRouter } from "next/navigation";
 import { getAnalysisErrorMessage } from "@/lib/question/analysis-error";
 import { getErrorMessage } from "@/lib/error-message";
@@ -54,6 +54,9 @@ export function useImageImportFlow(deps: ImageFlowDeps) {
     const [completedStages, setCompletedStages] = useState<ReturnType<typeof buildStageList>>([]);
     const preparedRef = useRef<PreparedImage | null>(null);
 
+    // 离开页面时释放解码后的原图与预览图；只在卸载时执行，重新渲染不能释放（裁剪还要用原图）
+    useEffect(() => () => preparedRef.current?.dispose(), []);
+
     const targetCtx = { fixedSubjectId, confirmBeforeImport, importSubjectId };
 
     const decodeErrorMessage = (error: ImageDecodeError) =>
@@ -210,6 +213,5 @@ export function useImageImportFlow(deps: ImageFlowDeps) {
         discardParts: () => setParts([]),
         closeBatch: () => setBatchImages(null),
         closeCropper: () => setIsCropperOpen(false),
-        disposePrepared: () => preparedRef.current?.dispose(),
     };
 }

src/lib/hooks/use-question-import.ts

@@ -68,9 +68,6 @@ export function useQuestionImport({ logPrefix, analyzeSubjectId, importSubjectId
         clearSaveError: () => saver.setSaveError(null),
     });
 
-    // 离开页面时释放解码后的原图与预览图
-    useEffect(() => () => imageFlow.disposePrepared(), [imageFlow]);
-
     useEffect(() => {
         fetchConfirmBeforeImport().then(setConfirmBeforeImport);
         apiClient.get<AppConfig>("/api/settings")