- 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
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