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)/shop/page.tsx

@@ -142,7 +142,7 @@ export default async function ShopPage({
 		: undefined;
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			{boughtMessage ? (
 				<div role="status" style={feedbackStyle("success")}>
 					{boughtMessage}
@@ -230,7 +230,7 @@ export default async function ShopPage({
 											borderRadius: "var(--radius-sm)",
 											display: "grid",
 											placeItems: "center",
-											background: "#f8fafc",
+											background: "var(--color-background)",
 											border: `2px solid ${a.color || "var(--border-color)"}`,
 											flexShrink: 0,
 											overflow: "hidden",

src/app/(site)/shop/topup/page.tsx

@@ -41,7 +41,7 @@ export default async function TopUpPage({
 	const rate = creditsPerUnit();
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			<ContentCard icon="💳" title={t("title")} subtitle={t("subtitle")} />
 
 			{sp.status === "cancel" ? (