Closes: [BS-3525](https://linear.app/affine-design/issue/BS-3525/markdown-adapter-紧跟着链接的-footnote-reference-会被识别成链接的一部分)
<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit
- **New Features**
- Improved handling of footnote references that immediately follow URLs in markdown, ensuring correct spacing and parsing.
- **Bug Fixes**
- Footnote references after URLs are now parsed correctly, preventing formatting issues.
- **Tests**
- Added comprehensive test suites to verify footnote reference preprocessing and markdown conversion.
- **Chores**
- Introduced Vitest as a development dependency and added a dedicated test configuration for the footnote module.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Closes: [BS-3272](https://linear.app/affine-design/issue/BS-3272/footnote-适配-exa-api-返回结果)
## What's Changed
Add link preview data support (favicon, title, description) for URL references in footnotes:
- Store and display URL preview data in footnotes
- Add encoding/decoding support for favicon URLs
- Optimize link preview by using existing preview data when available
donteatfriedrice