fix(contributors): prevent visitor-side GitHub requests when contributor data is unavailable

[castrojo]

Problem

fetch-contributors.js writes an empty object when no contributor requests succeed. PageContributors.tsx treats every missing key as a cache miss and requests commits from the visitor browser.

Evidence

With the current empty contributor map, contributorsData["docs/installation.md"] is undefined and the component falls back to an unauthenticated GitHub API request on each page footer.

Expected

Emit the standard unavailable payload for a failed complete fetch and suppress client-side API fallback for a globally unavailable dataset.