refactor(shadow): introduce shadow-card utility and extend tailwind-merge

app/(main)/post/[id]/page.tsx

@@ -30,7 +30,7 @@ export default async function PostDetailPage({ params }: { params: Promise<{ id:
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       </Link>
 
-      <Card className="hover:shadow-[var(--shadow-card)]">
+      <Card className="hover:shadow-card-hover">
         <CardHeader className="pb-3">
           <div className="flex items-center gap-3">
             <Avatar className="h-10 w-10">

app/(main)/post/new/page.tsx

@@ -101,7 +101,7 @@ export default function NewPostPage() {
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       </Link>
 
-      <Card className="hover:shadow-[var(--shadow-card)]">
+      <Card className="hover:shadow-card-hover">
         <CardHeader>
           <CardTitle className="tracking-tighter leading-tight">发布帖子</CardTitle>
         </CardHeader>