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)/marketplace/page.tsx

@@ -70,7 +70,7 @@ export default async function MarketplacePage() {
 	const total = offers.reduce((sum, o) => sum + o.price, 0);
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			<ContentCard icon="🛍️" title={t("title")} subtitle={t("subtitle")}>
 				<div className="stat-grid">
 					<StatBlock