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)/guilds/[id]/forum/[threadId]/page.tsx

@@ -159,7 +159,7 @@ export default async function GuildForumThreadPage({
 							: null;
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			{replied === "1" ? (
 				<div role="status" style={feedbackStyle("success")}>
 					{t("success.replied")}

src/app/(site)/guilds/[id]/forum/new/page.tsx

@@ -70,7 +70,7 @@ export default async function NewThreadPage({
 						: null;
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			{errorMessage ? (
 				<div role="alert" style={feedbackStyle("error")}>
 					{errorMessage}

src/app/(site)/guilds/[id]/forum/page.tsx

@@ -120,7 +120,7 @@ export default async function GuildForumPage({
 	const usernameById = new Map(users.map((u) => [u.id, u.username]));
 
 	return (
-		<main style={{ display: "grid", gap: "1.5rem" }}>
+		<main className="page-grid">
 			{posted === "1" ? (
 				<div role="status" style={feedbackStyle("success")}>
 					{t("success.posted")}