style(admin): 后台各页容器统一居中 (mx-auto)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

admin-frontend/src/components/ApiKeysPage.tsx

@@ -44,7 +44,7 @@ export default function ApiKeysPage() {
   }
 
   return (
-    <div className="max-w-4xl">
+    <div className="mx-auto max-w-4xl">
       <div className="flex items-center justify-between mb-4">
         <div>
           <h2 className="text-lg font-semibold text-gray-800 flex items-center gap-2">

admin-frontend/src/components/AuditLogPage.tsx

@@ -40,7 +40,7 @@ export default function AuditLogPage() {
   const pages = Math.max(1, Math.ceil(total / size));
 
   return (
-    <div className="max-w-5xl">
+    <div className="mx-auto max-w-5xl">
       <h2 className="mb-4 flex items-center gap-2 text-lg font-semibold text-gray-800">
         <ScrollText className="h-5 w-5 text-emerald-600" /> 操作日志
         <span className="text-sm font-normal text-gray-400">共 {total} 条</span>

admin-frontend/src/components/BrandsPage.tsx

@@ -105,7 +105,7 @@ export default function BrandsPage() {
   }
 
   return (
-    <div className="max-w-4xl">
+    <div className="mx-auto max-w-4xl">
       <div className="flex items-center justify-between mb-4">
         <div>
           <h2 className="text-lg font-semibold text-gray-800 flex items-center gap-2">

admin-frontend/src/components/CategoriesPage.tsx

@@ -55,7 +55,7 @@ export default function CategoriesPage() {
   }
 
   return (
-    <div className="max-w-5xl">
+    <div className="mx-auto max-w-5xl">
       <div className="flex items-center justify-between mb-4">
         <div>
           <h2 className="text-lg font-semibold text-gray-800 flex items-center gap-2">

admin-frontend/src/components/StatsPage.tsx

@@ -44,7 +44,7 @@ export default function StatsPage() {
   }, []);
 
   return (
-    <div className="max-w-5xl">
+    <div className="mx-auto max-w-5xl">
       <h2 className="mb-4 flex items-center gap-2 text-lg font-semibold text-gray-800">
         <BarChart3 className="h-5 w-5 text-emerald-600" /> 数据概览
       </h2>

admin-frontend/src/components/SubmissionsPage.tsx

@@ -56,7 +56,7 @@ export default function SubmissionsPage({ onPending }: { onPending?: (n: number)
   }
 
   return (
-    <div>
+    <div className="mx-auto max-w-5xl">
       <div className="flex items-center gap-2 mb-4">
         {STATUS_TABS.map((t) => (
           <button
@@ -193,7 +193,7 @@ function SubmissionView({ id, onBack }: { id: string; onBack: () => void }) {
   const nutri = Object.entries(p.nutriments || {});
 
   return (
-    <div className="max-w-3xl">
+    <div className="mx-auto max-w-3xl">
       <button onClick={onBack} className="text-sm text-gray-500 flex items-center gap-1 mb-4">
         <ArrowLeft className="h-4 w-4" /> 返回投稿队列
       </button>