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/categories/CategoryForm.tsx

@@ -21,6 +21,7 @@ export default function CategoryForm({
   })
   const [loading, setLoading] = useState(false)
   const [error, setError] = useState("")
+  const [success, setSuccess] = useState(false)
 
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault()
@@ -41,9 +42,12 @@ export default function CategoryForm({
       })
 
       if (res.ok) {
+        setSuccess(true)
+        await new Promise(r => setTimeout(r, 1000))
         router.push("/admin/categories")
       } else {
-        setError("保存失败")
+        const data = await res.json().catch(() => ({}))
+        setError(data.error || "保存失败")
       }
     } catch (err) {
       setError("保存失败，请稍后重试")
@@ -100,10 +104,22 @@ export default function CategoryForm({
         />
       </div>
 
+      {success && (
+        <div className="bg-green-50 border border-green-200 text-green-700 px-4 py-3 rounded-lg text-sm">
+          保存成功
+        </div>
+      )}
+
+      {error && (
+        <div className="bg-red-50 border border-red-200 text-red-700 px-4 py-3 rounded-lg text-sm">
+          {error}
+        </div>
+      )}
+
       <div className="flex gap-4">
         <button
           type="submit"
-          disabled={loading}
+          disabled={loading || success}
           className="bg-blue-600 text-white px-6 py-2 rounded-lg hover:bg-blue-700 disabled:bg-blue-400"
         >
           {loading ? "保存中..." : "保存"}