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/page.tsx

@@ -4,7 +4,9 @@ import Image from "next/image"
 
 export default async function HomePage() {
   const agents = await prisma.agent.findMany({
+    where: { isFeatured: true },
     include: { category: true },
+    orderBy: { featuredOrder: "asc" },
     take: 6,
   })