feat: add agent favorite functionality with UI and API implementation

- Add `addFavorite` and `removeFavorite` API functions in agent.js
- Implement favorite button UI in Agents.vue with star icons
- Add login check before favorite operations using token manager
- Sort agents with favorites appearing at the top of the list
- Include success/error messages for user feedback
- Add backend API endpoints for creating/deleting agent favorites
- Update agent list response to include favorite status
- Style favorite button with hover and active states

frontend/app/web-gold/src/api/agent.js

@@ -116,3 +116,27 @@ export function getMessages(params) {
     params
   })
 }
+
+/**
+ * 添加智能体收藏
+ * @param {number} agentId - 智能体ID
+ */
+export function addFavorite(agentId) {
+  return request({
+    url: `${BASE_URL}/agent/favorite/create`,
+    method: 'post',
+    params: { agentId }
+  })
+}
+
+/**
+ * 取消智能体收藏
+ * @param {number} agentId - 智能体ID
+ */
+export function removeFavorite(agentId) {
+  return request({
+    url: `${BASE_URL}/agent/favorite/delete`,
+    method: 'delete',
+    params: { agentId }
+  })
+}