feat(knowledge): 添加知识库标签统计功能

- 在 KnowledgeItemApplicationService 中注入 TagMapper 并调用统计方法
- 新增 countKnowledgeSetTags 方法用于计算知识库中的标签总数
- 在 KnowledgeManagementStatisticsResponse 中添加 totalTags 字段
- 在前端 KnowledgeManagementPage 中显示标签总数统计信息
- 更新统计卡片布局从 3 列改为 4 列以适应新增统计项
- 在知识管理模型中添加 totalTags 类型定义

frontend/src/pages/KnowledgeManagement/Home/KnowledgeManagementPage.tsx

@@ -40,6 +40,10 @@ const DEFAULT_STATISTICS: StatisticsItem[] = [
     title: "文件总数",
     value: 0,
   },
+  {
+    title: "标签总数",
+    value: 0,
+  },
   {
     title: "总大小",
     value: "0 B",
@@ -113,6 +117,10 @@ export default function KnowledgeManagementPage() {
           title: "文件总数",
           value: stats?.totalFiles ?? 0,
         },
+        {
+          title: "标签总数",
+          value: stats?.totalTags ?? 0,
+        },
         {
           title: "总大小",
           value: formatBytes(stats?.totalSize ?? 0),
@@ -276,7 +284,7 @@ export default function KnowledgeManagementPage() {
 
       <div className="grid grid-cols-1 gap-4">
         <Card>
-          <div className="grid grid-cols-3">
+          <div className="grid grid-cols-4">
             {statisticsData.map((item) => (
               <Statistic
                 title={item.title}

frontend/src/pages/KnowledgeManagement/knowledge-management.model.ts

@@ -82,6 +82,7 @@ export interface KnowledgeManagementStatistics {
   totalKnowledgeSets: number;
   totalFiles: number;
   totalSize: number;
+  totalTags: number;
 }
 
 export interface KnowledgeItemSearchResult {