Promote GitHub button to live-stars pill (matches /skill page)

The top-bar GitHub icon-btn is now the same icon-btn-pill that
already ships on /skill — same component, same loading state,
same fetch-from-cache flow.

- Pill renders a placeholder "—" with data-loading="true", then
  fetches https://api.github.com/repos/Jakubantalik/transitions.dev
  once per page visit and writes the formatted star count
  ("12", "1.2k", "12k") into the label.
- Result is cached in sessionStorage (5 min TTL) under the same
  "tdev:gh-stars" key as /skill so soft-navigations don't re-hit
  the API and the count paints from cache instantly.
- Falls back silently to "Star" on network / rate-limit errors;
  the link itself always works.
- Mobile rule: the Skill pill still collapses to a 36px icon to
  keep the row compact, but #gh-stars-btn keeps its label so the
  live count stays visible — that's the metric people came for.
- Added the .icon-btn-pill[data-loading="true"] .pill-label dim
  rule so the placeholder doesn't visually "pop" wider when the
  real count arrives.

Co-authored-by: Cursor <cursoragent@cursor.com>