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",