fix: 优化成就页头部布局适配

将成就页头部容器由响应式flex方向调整为始终横向排列，统一各端显示效果，提升布局一致性。

src/views/AchievementsView.vue

@@ -1,6 +1,6 @@
 <template>
   <div class="container mx-auto p-4 sm:p-6 space-y-4 sm:space-y-6">
-    <div class="flex flex-col sm:flex-row sm:items-center sm:justify-between gap-4">
+    <div class="flex flex-row items-center justify-between gap-4">
       <h1 class="text-2xl sm:text-3xl font-bold">{{ t('achievements.title') }}</h1>
       <div class="flex items-center gap-2">{{ unlockedCount }} / {{ totalCount }} {{ t('achievements.unlocked') }}</div>
     </div>