fix(exams): fix composite question sub-questions not showing in preview

- Fix buildQuestion in editor-to-structure.ts: recursively detect nested
  questionBlock nodes as subQuestions (composite questions now properly
  extract child questions instead of treating them as plain text)
- Fix buildQuestionBlock in actions.ts: AI auto-mark now generates nested
  questionBlock nodes for sub-questions instead of plain paragraphs, so
  they are properly structured and detectable by the parser
- Rewrite ExamPreview component with proper layout:
  - Question header: number + type label badge + score
  - Indented question text and options
  - Composite sub-questions shown in nested block with left border
  - Image thumbnails
  - Empty state message
  - Title centered at top
  - Group titles with bottom border

src/modules/exams/actions.ts

@@ -1137,17 +1137,28 @@ const buildTiptapDocFromAiResponse = (data: unknown): unknown => {
       })
     }
 
-    // 子题(阅读理解的小题)
+    // 子题(阅读理解的小题)—— 生成为嵌套的 questionBlock,便于编辑器层级展示与解析
     const subQuestions = Array.isArray(contentNode.subQuestions)
       ? contentNode.subQuestions
       : []
     for (const sub of subQuestions) {
       if (!isRecord(sub)) continue
       const subText = typeof sub.text === "string" ? sub.text : ""
+      const subScore = typeof sub.score === "number" ? sub.score : 0
       if (subText) {
+        // 子题文本按行拆分为段落
+        const subLines = subText.split("\n").filter((l) => l.trim().length > 0)
+        const subInner =
+          subLines.length > 0
+            ? subLines.map((line) => ({
+                type: "paragraph",
+                content: [{ type: "text", text: line }],
+              }))
+            : [{ type: "paragraph", content: [{ type: "text", text: " " }] }]
         inner.push({
-          type: "paragraph",
-          content: [{ type: "text", text: subText }],
+          type: "questionBlock",
+          attrs: { questionId: "", type: "text", score: subScore },
+          content: subInner,
         })
       }
     }