Add commit SHA and build date to website footer

- Display short commit SHA with link to GitHub commit
- Show build date in YYYY-MM-DD format
- Use GITHUB_SHA env var in CI, fallback to git command locally
- Add subtle styling for build info text

website/public/styles/global.css

@@ -688,6 +688,12 @@ a:hover {
   color: var(--color-accent);
 }
 
+.site-footer .build-info {
+  margin-top: 8px;
+  font-size: 12px;
+  opacity: 0.7;
+}
+
 /* Buttons */
 .btn {
   display: inline-flex;