feat: Question Bank Reference for course generation

## Summary

Add a **Question Bank Reference** option in the Advanced Settings panel during course generation. This allows users to provide reference questions (via file upload or pasted text), which the AI agents can then use to:

- **Directly include** selected questions from the reference bank
- **Reference the question style/format** to generate new, similarly structured questions

## Motivation

Educators often have existing question banks, past exams, or curated problem sets that reflect their desired difficulty level, topic coverage, and question format. Currently, OpenMAIC generates all questions from scratch, which means the output may not align with the instructor's established assessment style.

By allowing users to provide reference questions, the AI can:
- Maintain consistency with existing course materials
- Match the expected difficulty and format of real exams
- Save time by reusing proven, well-crafted questions where appropriate

## Proposed Design

### Input
- A new section in **Advanced Settings** (alongside existing options like web search)
- Supports:
- **File upload** (PDF, TXT, MD — reusing existing upload infrastructure)
- **Paste text** directly into a text area
- Multiple reference sources allowed

### Behavior
- When reference questions are provided, they are injected as additional context into the AI generation prompt
- The AI naturally decides whether to draw directly from the bank or use the questions as stylistic/structural references
- No complex mode selection needed — the AI handles it intelligently based on the provided material

### UI Placement
- Within the generation page's advanced/optional configuration area
- Toggle to enable/disable the feature
- Clear indicator showing how many reference questions/files are loaded

## Additional Context

This feature complements the existing web search integration — while web search provides broad topic context, question bank reference provides specific assessment-level guidance.