chore(channels): drop admin-side available channels view

Remove the admin-side "Available Channels" aggregate view — admins
already see full channel configuration (groups, pricing, model
mappings) in the channel edit dialog, making a read-only admin
aggregate view redundant. The user-side "可用渠道" remains.

Backend:
- Delete handler/admin/available_channel_handler.go (+ test)
- Drop AdminHandlers.AvailableChannel field and wire injection
- Remove /admin/channels/available route

Frontend:
- Delete views/admin/AvailableChannelsView.vue
- Drop /admin/available-channels router entry
- Strip AvailableChannel types + listAvailable from api/admin/channels.ts

backend/internal/server/routes/admin.go

@@ -560,7 +560,6 @@ func registerChannelRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 	channels := admin.Group("/channels")
 	{
 		channels.GET("", h.Admin.Channel.List)
-		channels.GET("/available", h.Admin.AvailableChannel.List)
 		channels.GET("/model-pricing", h.Admin.Channel.GetModelDefaultPricing)
 		channels.GET("/:id", h.Admin.Channel.GetByID)
 		channels.POST("", h.Admin.Channel.Create)