feat: add save feedback prompts, featured agent fields, dynamic Dify API config, and chat improvements

- Add success/error feedback messages near submit buttons in all admin forms
- Display success prompt for 1s before redirect after save
- Show API error details on save failure
- Add isFeatured/featuredOrder fields to Agent model and admin UI
- Add difyApiUrl/difyApiKey fields for per-agent Dify API configuration
- Show featured badge column in agent admin list
- Display featured agents on homepage sorted by order
- Refactor chat page streaming with AbortController and stable userId
- Improve Dify API proxy to use per-agent credentials

app/admin/agents/page.tsx

@@ -39,6 +39,7 @@ export default async function AdminAgentsPage() {
               <tr>
                 <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">名称</th>
                 <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">分类</th>
+                <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">热门</th>
                 <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">状态</th>
                 <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">使用次数</th>
                 <th className="px-6 py-3 text-right text-xs font-medium text-gray-500 uppercase">操作</th>
@@ -59,6 +60,15 @@ export default async function AdminAgentsPage() {
                   <td className="px-6 py-4 text-sm text-gray-600">
                     {agent.category?.name || "-"}
                   </td>
+                  <td className="px-6 py-4">
+                    {agent.isFeatured ? (
+                      <span className="inline-flex px-2 py-1 text-xs font-medium rounded-full bg-orange-100 text-orange-700">
+                        热门
+                      </span>
+                    ) : (
+                      <span className="text-sm text-gray-400">-</span>
+                    )}
+                  </td>
                   <td className="px-6 py-4">
                     <span className={`inline-flex px-2 py-1 text-xs font-medium rounded-full ${
                       agent.status === "active"