feat: add question bank reference for course generation

[YizukiAme]

Summary

Adds a Question Bank Reference feature to the generation toolbar, allowing users to upload or paste reference questions that the AI uses when generating quiz scenes. Closes #43.

What it does

• 📖 Question Bank pill button in the toolbar with a popover UI containing:
  • Textarea for pasting reference questions
  • File import button (.txt / .md)
  • Clear button and character count indicator
  • Active state pill when content is loaded
• Prompt injection: Question bank text flows through the full pipeline:
  FormState → sessionStorage → generation-preview → scene-outlines-stream API → prompt template
• AI behavior: When a question bank is provided, the AI can either:
  • Draw original questions directly from the bank
  • Generate new questions following the same style, format, and difficulty

Files changed (7)

File	Change
lib/i18n/common.ts	7 new i18n keys (zh-CN + en-US)
app/generation-preview/types.ts	questionBankContext field in session state
lib/generation/prompts/templates/.../user.md	### Question Bank Reference section + instruction #9
app/api/generate/scene-outlines-stream/route.ts	Accept & pass questionBankContext to prompt
app/generation-preview/page.tsx	Pass questionBankContext to API
app/page.tsx	questionBankText in FormState + session
components/generation/generation-toolbar.tsx	Question Bank popover UI

Testing

• ✅ pnpm build passes
• ✅ Tested with a full exam paper → AI generates questions in similar style
• ✅ Tested with a single question → AI directly references the original question
• ✅ File import (.txt/.md) and paste both work
• ✅ Clear button and active state indicator work correctly