✨ feat: implement backend channel duplication & streamline frontend copy flow

Add a dedicated backend endpoint to clone an existing channel (including its key) and
replace all previous front-end cloning logic with a single API call.

Backend
• controller/channel.go
  – add CopyChannel: safely clone a channel, reset balance/usage, append name suffix,
    preserve key, create abilities, return new ID.
  – supports optional query params: `suffix`, `reset_balance`.
• router/api-router.go
  – register POST /api/channel/copy/:id (secured by AdminAuth).
• model interaction uses BatchInsertChannels to ensure transactional integrity.

Frontend
• ChannelsTable.js
  – simplify copySelectedChannel: call /api/channel/copy/{id} and refresh list.
  – remove complex field-manipulation & key-fetching logic.
  – improved error handling.

Security & stability
• All cloning done server-side; sensitive key never exposed to client.
• Route inherits existing admin middleware.
• Graceful JSON responses with detailed error messages.

router/api-router.go

@@ -115,6 +115,7 @@ func SetApiRouter(router *gin.Engine) {
 			channelRoute.POST("/fetch_models", controller.FetchModels)
 			channelRoute.POST("/batch/tag", controller.BatchSetChannelTag)
 			channelRoute.GET("/tag/models", controller.GetTagModels)
+			channelRoute.POST("/copy/:id", controller.CopyChannel)
 		}
 		tokenRoute := apiRouter.Group("/token")
 		tokenRoute.Use(middleware.UserAuth())