refactor(exams): redesign exam creation page with 3-mode selector

- Replace cramped 3-column grid with vertical layout
- Add 3 large selectable cards at top: Manual / AI / Rich Text Editor
- Rich Text Editor mode redirects to /teacher/exams/new
- Basic info form is now always visible (not hidden in AI mode)
- Exam mode config always visible at bottom
- Add "rich" to mode enum with validation bypass
- Replace all hardcoded English/Chinese strings with i18n keys
- Add 20+ new i18n keys to zh-CN and en (mode labels, descriptions, actions)
- Clean up mixed-language UI text

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

@@ -32,7 +32,32 @@
       "createFailed": "创建考试失败",
       "loadFormFailed": "加载表单数据失败",
       "loadSubjectsFailed": "加载科目失败",
-      "loadGradesFailed": "加载年级失败"
+      "loadGradesFailed": "加载年级失败",
+      "creating": "创建中...",
+      "detailsTitle": "基本信息",
+      "detailsDesc": "设置考试的核心信息。",
+      "titlePlaceholder": "例如:期中数学考试",
+      "subjectPlaceholder": "选择科目",
+      "gradePlaceholder": "选择年级",
+      "difficultyPlaceholder": "选择难度",
+      "difficultyLevel1": "1级 (简单)",
+      "difficultyLevel2": "2级",
+      "difficultyLevel3": "3级 (中等)",
+      "difficultyLevel4": "4级",
+      "difficultyLevel5": "5级 (困难)",
+      "loadingSubjects": "加载科目中...",
+      "loadingGrades": "加载年级中...",
+      "scheduledAtDesc": "设置后,考试将在指定时间开放。",
+      "modeManual": "手动组卷",
+      "modeManualDesc": "从题库手动选择题目并组织试卷结构。",
+      "modeManualAction": "创建并开始组卷",
+      "modeAi": "AI 生成",
+      "modeAiDesc": "粘贴试卷文本,AI 自动解析题目并生成结构化预览。",
+      "modeAiAction": "后台生成试卷",
+      "modeRich": "富文本编辑器",
+      "modeRichDesc": "使用富文本编辑器粘贴试卷文本,手动或 AI 辅助标记题目、分组、填空等。",
+      "modeRichHint": "富文本编辑器支持选中文本标记题目/分组/加点字/填空,右侧实时预览试卷效果。",
+      "modeRichAction": "打开富文本编辑器"
     },
     "status": {
       "draft": "草稿",