* Add GitHub star count to site header
Fetches star count from GitHub API via /api/github-stars with 5-minute
server-side caching (ISR + stale-while-revalidate). Shows formatted
count (e.g. "2.2k") next to the GitHub link in both desktop nav and
mobile drawer.
* Move star count to separate badge left of download button
GitHub icon + formatted count as its own clickable element in the
right header section, separate from the nav links. Desktop only.
* Center GitHub stars badge vertically in header
* Add right padding to GitHub stars badge
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