Add popularity sorting using GitHub stars

[akuligowski9]

Many APIs in the catalog have GitHub repositories. Using their star counts as a popularity signal would allow sorting by community adoption.

Proposed approach

1. For each API in catalog.json that links to a GitHub repo, fetch the star count via GitHub API
2. Store the data in a static JSON file (updated periodically, not at runtime)
3. Add a 'Sort by popularity' option to the grid

Considerations

• Many APIs don't have GitHub repos so this would be partial coverage
• GitHub API rate limits need to be respected
• A script to refresh the data periodically would be needed

Acceptance criteria

• Popularity sort option visible in the grid
• APIs with no star data fall to the end
• Static data file checked into the repo with a refresh script