fix: 修复移动端Header链接和添加scoped样式

- 移动端Header品牌链接指向/mobile而非根路径
- 为桌面端Header样式添加scoped属性避免样式污染

app/components/Header.vue

@@ -69,7 +69,7 @@ watchEffect(() => {
   lastScrollY.value = scrollY.value
 })
 </script>
-<style lang="scss">
+<style lang="scss" scoped>
 .header {
   box-sizing: border-box;
   padding: 22px 0;

app/components/mobile/Header.vue

@@ -1,7 +1,7 @@
 <template>
   <header class="mb-header">
     <div class="mb-header-container" :style="{ height: headerHeight }">
-      <a href="/" class="brand">
+      <a href="/mobile" class="brand">
         <NuxtImg src="/logo.png" alt="DDAC logo" class="brand-logo" width="14" height="14" />
         <h1 class="brand-title">DDAC</h1>
       </a>