style: 留言板SN标签改用等宽字体更醒目

frontend/src/pages/admin/AdminDashboard.tsx

@@ -128,9 +128,9 @@ function MsgRow({ m }: { m: ProductMessageItem }) {
           </div>
           <p className="mb-2 text-sm leading-relaxed text-gray-600">{m.content}</p>
           <div className="flex items-center gap-2">
-            <Tag color="blue" className="m-0 text-xs">{m.product_sn}</Tag>
+            <Tag color="blue" className="m-0 text-xs font-mono">{m.product_sn}</Tag>
             {m.material_name && (
-              <span className="truncate text-xs text-gray-400">{m.material_name}</span>
+              <span className="truncate text-xs text-gray-500">{m.material_name}</span>
             )}
           </div>
         </div>