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/api/admin/agents/route.ts

@@ -12,10 +12,14 @@ export async function POST(request: NextRequest) {
         description: data.description,
         icon: data.icon || null,
         categoryId: data.categoryId ? parseInt(data.categoryId) : null,
+        difyApiUrl: data.difyApiUrl || null,
+        difyApiKey: data.difyApiKey || null,
         features: data.features || "",
         hotQuestions: data.hotQuestions || "[]",
         quickQuestions: data.quickQuestions || "[]",
         status: data.status || "active",
+        isFeatured: data.isFeatured ?? false,
+        featuredOrder: data.featuredOrder ?? 0,
       },
     })
     return NextResponse.json(agent)