Closes: [BS-3113](https://linear.app/affine-design/issue/BS-3113/footnote-popup-ui-更新)
<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit
- **New Features**
- Added a description display below the chip in the footnote popup when available.
- **Style**
- Improved the appearance of footnote popups with enhanced border radius, padding, and layout.
- Updated font styling for labels and descriptions.
- Simplified chip styling by reducing height and removing unnecessary CSS properties.
- **Bug Fixes**
- Tooltip content for URL references now displays both the link preview title and URL when available.
- **Refactor**
- Simplified click handling by attaching it to the container instead of individual elements.
- Removed unused click event properties from the footnote popup chip.
<!-- 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
