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/en/exam-homework.json

@@ -32,7 +32,32 @@
       "createFailed": "Failed to create exam",
       "loadFormFailed": "Failed to load form data",
       "loadSubjectsFailed": "Failed to load subjects",
-      "loadGradesFailed": "Failed to load grades"
+      "loadGradesFailed": "Failed to load grades",
+      "creating": "Creating...",
+      "detailsTitle": "Exam Details",
+      "detailsDesc": "Define the core information for your exam.",
+      "titlePlaceholder": "e.g. Midterm Mathematics Exam",
+      "subjectPlaceholder": "Select subject",
+      "gradePlaceholder": "Select grade level",
+      "difficultyPlaceholder": "Select level",
+      "difficultyLevel1": "Level 1 (Easy)",
+      "difficultyLevel2": "Level 2",
+      "difficultyLevel3": "Level 3 (Medium)",
+      "difficultyLevel4": "Level 4",
+      "difficultyLevel5": "Level 5 (Hard)",
+      "loadingSubjects": "Loading subjects...",
+      "loadingGrades": "Loading grades...",
+      "scheduledAtDesc": "If set, this exam will be scheduled for a specific time.",
+      "modeManual": "Manual Assembly",
+      "modeManualDesc": "Manually select questions from the bank and organize structure.",
+      "modeManualAction": "Create & Start Building",
+      "modeAi": "AI Generation",
+      "modeAiDesc": "Paste exam text and AI will auto-parse questions into structured preview.",
+      "modeAiAction": "Generate in Background",
+      "modeRich": "Rich Text Editor",
+      "modeRichDesc": "Use rich text editor to paste exam text, manually or AI-assisted marking of questions, groups, blanks, etc.",
+      "modeRichHint": "The rich text editor supports selecting text to mark as questions/groups/dotted chars/blanks, with live preview on the right.",
+      "modeRichAction": "Open Rich Text Editor"
     },
     "status": {
       "draft": "Draft",

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",