Improve accessibility and polish page styling

app/leaderboard/loading.tsx

@@ -6,6 +6,8 @@ export default async function LoadingLeaderboard() {
       className="section page-content leaderboard-page"
       aria-busy="true"
       aria-label={t("Loading leaderboard")}
+      id="main-content"
+      tabIndex={-1}
     >
       <span className="eyebrow">{t("THE EXPLORERS’ BOARD")}</span>
       <h1>{t("Every block counts.")}</h1>