feat(exams): add section/group structure nodes with auto stats

Distinguish structural levels from questions:
- sectionBlock (new): top-level volume like "第Ⅰ卷 选择题(共24分)"
- groupBlock (enhanced): section like "一、选择题" with instruction field
- questionBlock (composite): reserved for reading comprehension

Key changes:
- New section-block.tsx extension with level attr (1=卷, 2=部分) and
  auto-computed question count + total score in NodeView
- group-block.tsx: add instruction field ("每小题3分"), auto stats display
- editor-to-structure.ts: recursive buildStructureNode supports arbitrary
  nesting (section > group > question), computeStats accumulates scores
- exam-rich-form.tsx ExamPreview: render section/group/question with
  distinct styles and stats badges
- selection-toolbar.tsx: add "分卷" button (Layers icon)
- exam-rich-editor.tsx: register SectionBlock, expose insertSection/
  wrapInSection via ref
- actions.ts: AI prompt now outputs volumes[] + groups[] structure with
  instruction; buildTiptapDocFromAiResponse generates nested sectionBlock
- i18n: add markSection keys (zh-CN/en)

Structural nodes are NOT questions: their question count and total score
are automatically computed from child questions, not manually set.

src/shared/i18n/messages/zh-CN/exam-homework.json

@@ -166,7 +166,8 @@
       "emptyEditor": "请在左侧编辑区输入或粘贴试卷内容",
       "emptyPreview": "预览将在此处显示",
       "markQuestion": "标记题目",
-      "markGroup": "标记分组",
+      "markGroup": "标记大题",
+      "markSection": "标记分卷",
       "markDotted": "加点字",
       "markBlank": "填空",
       "insertImage": "插入图片",