feat: 增强 API Key 标签选择功能

- 添加获取已存在标签的 API 端点 /admin/api-keys/tags
- 重构标签输入 UI，从下拉菜单改为平铺展示
- 支持点击选择已有标签，避免重复创建
- 增加弹框宽度 (max-w-4xl)，改善布局和用户体验
- 统一创建和编辑 API Key 的标签管理体验

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

web/admin-spa/src/stores/apiKeys.js

@@ -159,6 +159,21 @@ export const useApiKeysStore = defineStore('apiKeys', () => {
     }
   }
   
+  // 获取已存在的标签
+  const fetchTags = async () => {
+    try {
+      const response = await apiClient.get('/admin/api-keys/tags')
+      if (response.success) {
+        return response.data || []
+      } else {
+        throw new Error(response.message || '获取标签失败')
+      }
+    } catch (err) {
+      console.error('获取标签失败:', err)
+      return []
+    }
+  }
+  
   // 重置store
   const reset = () => {
     apiKeys.value = []
@@ -186,6 +201,7 @@ export const useApiKeysStore = defineStore('apiKeys', () => {
     renewApiKey,
     deleteApiKey,
     fetchApiKeyStats,
+    fetchTags,
     sortApiKeys,
     reset
   }