Add search and filter within generated digest

[akuligowski9]

Summary

After generating a digest with many papers across multiple disciplines, there's no way to search or filter without regenerating. Add client-side search/filter to find papers by keyword.

What to do

1. Add a search input above the digest content (visible only when digest has content)
2. Filter paper cards in real-time as the user types (Alpine.js recommended — no server round-trip)
3. Match against paper title and abstract text
4. Show/hide entire source sections if no papers match
5. Show a "no results" message if nothing matches
6. Clear button to reset the filter

Notes

• This should be purely client-side (Alpine.js) for instant feedback
• The digest data is already rendered in the DOM — just show/hide elements
• Consider debouncing the search input for performance with many papers