style: 平台管理端改为浅色主题，与企业端配色一致

frontend/src/App.tsx

@@ -84,19 +84,19 @@ function PlatformRoute({ children }: { children: React.ReactNode }) {
 function PlatformLayout({ children }: { children: React.ReactNode }) {
   const [sidebarOpen, setSidebarOpen] = useState(false)
   return (
-    <div className="flex min-h-screen bg-slate-100">
+    <div className="flex min-h-screen bg-[#f8f9fb]">
       <PlatformSidebar mobileOpen={sidebarOpen} onClose={() => setSidebarOpen(false)} />
       <div className="flex-1 flex flex-col min-w-0">
-        <header className="h-14 bg-slate-950 border-b border-slate-800 flex items-center px-4 shrink-0">
+        <header className="h-14 bg-white border-b border-gray-200 flex items-center px-4 shrink-0">
           <button
-            className="md:hidden mr-3 text-slate-400"
+            className="md:hidden mr-3 text-gray-400"
             onClick={() => setSidebarOpen(true)}
             aria-label="打开菜单"
           >
             <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2"><line x1="3" y1="6" x2="21" y2="6"/><line x1="3" y1="12" x2="21" y2="12"/><line x1="3" y1="18" x2="21" y2="18"/></svg>
           </button>
-          <span className="text-sm text-slate-400">平台管理后台</span>
-          <span className="ml-2 px-2 py-0.5 rounded text-[10px] font-bold bg-amber-400 text-slate-950">ADMIN</span>
+          <span className="text-sm text-gray-500">平台管理后台</span>
+          <span className="ml-2 px-2 py-0.5 rounded text-[10px] font-bold bg-primary text-white">ADMIN</span>
         </header>
         <main className="flex-1 py-6 px-4 md:px-6">
           <div className="max-w-7xl mx-auto">