Polish content pages: share AuthTopBar, page-grid helper and responsive tables

- Extract shared AuthTopBar frosted top bar; reuse in login & register
- Add .page-grid class replacing 48 inline grid styles
- Add .table-scroll + .table-cell-truncate helpers; wrap leaderboard/staff tables
- Constrain settings page width (max-w-5xl) and stack profile card on mobile
- Use theme background var for shop category icon (dark-mode safe)

src/app/(site)/radio/giveaways/[id]/page.tsx

@@ -38,7 +38,7 @@ export default async function RadioGiveawayDetailPage({
 			: null);
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			<p className="muted" style={{ margin: 0 }}>
 				<Link href="/radio/giveaways">← Back to giveaways</Link>
 			</p>

src/app/(site)/radio/giveaways/page.tsx

@@ -25,7 +25,7 @@ export default async function RadioGiveawaysPage() {
 		.catch(() => []);
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			<ContentCard icon="🎁" title={t("title")} subtitle={t("subtitle")} />
 
 			<ContentCard padded={giveaways.length === 0}>