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>

app/leaderboard/page.tsx

@@ -42,7 +42,7 @@ export default async function LeaderboardPage({
   return (
     <>
       <Header />
-      <main className="section page-content leaderboard-page" id="main-content">
+      <main className="section page-content leaderboard-page" id="main-content" tabIndex={-1}>
         <Link href="/" className="text-link leaderboard-back">
           <ArrowLeft size={14} /> {t("Back to home")}
         </Link>