refactor(projects): 路由声明重构，添加composables，BaseLayout进行中，文件夹规范

src/layouts/BaseLayout/components/common/HeaderPlaceholder/index.vue

@@ -0,0 +1,12 @@
+<template>
+  <div v-if="theme.fixedHeaderAndTab" :style="{ height: headerAndMultiTabHeight }"></div>
+</template>
+
+<script setup lang="ts">
+import { useThemeStore } from '@/store';
+import { useLayoutConfig } from '@/composables';
+
+const theme = useThemeStore();
+const { headerAndMultiTabHeight } = useLayoutConfig();
+</script>
+<style scoped></style>