feat: 实现通用组件

顶部栏、侧边栏、分页表格、Markdown 渲染、模型选择对话框、模型状态标签、页面卡片、占位视图等八个公共组件，及 Font Awesome 图标资源。

frontend/src/components/PlaceholderView.vue

@@ -0,0 +1,52 @@
+<script setup lang="ts">
+/**
+ * 占位页面
+ * 用于尚未实现的菜单项，保持统一的卡片样式与「敬请期待」提示
+ */
+import PageCard from '@/components/PageCard.vue'
+
+defineProps<{
+  title?: string
+  desc?: string
+}>()
+</script>
+
+<template>
+  <PageCard :title="title">
+    <div class="placeholder">
+      <i class="fa fa-wrench placeholder-icon" />
+      <p class="placeholder-title">{{ title }} 功能开发中</p>
+      <p class="placeholder-desc">{{ desc || '敬请期待，我们正在努力构建该功能。' }}</p>
+    </div>
+  </PageCard>
+</template>
+
+<style scoped lang="scss">
+.placeholder {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  padding: 80px 20px;
+  text-align: center;
+
+  .placeholder-icon {
+    font-size: 56px;
+    color: #c0c4cc;
+    margin-bottom: 20px;
+  }
+
+  .placeholder-title {
+    font-size: 18px;
+    font-weight: 500;
+    color: #303133;
+    margin: 0 0 8px;
+  }
+
+  .placeholder-desc {
+    font-size: 14px;
+    color: #909399;
+    margin: 0;
+  }
+}
+</style>