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

prisma/schema.prisma

@@ -39,8 +39,12 @@ model Agent {
   features         String         @default("")
   hotQuestions     String         @default("[]")
   quickQuestions   String         @default("[]")
+  difyApiUrl       String?
+  difyApiKey       String?
   usageCount       Int            @default(0)
   status           String         @default("active")
+  isFeatured       Boolean        @default(false)
+  featuredOrder    Int            @default(0)
   createdAt     DateTime       @default(now())
   updatedAt     DateTime       @updatedAt
   category      Category?      @relation(fields: [categoryId], references: [id])