Add gistpreview.github.io support to search feature

When hosted on gistpreview.github.io, the search feature now:
- Detects the gistpreview hostname
- Extracts the gist ID from the URL path
- Fetches gist metadata from GitHub API to get the owner
- Constructs raw gist URLs for fetching page files

This allows search to work correctly when viewing transcripts via gistpreview.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>