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/handler/wire.go

@@ -36,7 +36,6 @@ func ProvideAdminHandlers(
 	channelHandler *admin.ChannelHandler,
 	channelMonitorHandler *admin.ChannelMonitorHandler,
 	channelMonitorTemplateHandler *admin.ChannelMonitorRequestTemplateHandler,
-	availableChannelHandler *admin.AvailableChannelHandler,
 	paymentHandler *admin.PaymentHandler,
 ) *AdminHandlers {
 	return &AdminHandlers{
@@ -67,7 +66,6 @@ func ProvideAdminHandlers(
 		Channel:                channelHandler,
 		ChannelMonitor:         channelMonitorHandler,
 		ChannelMonitorTemplate: channelMonitorTemplateHandler,
-		AvailableChannel:       availableChannelHandler,
 		Payment:                paymentHandler,
 	}
 }
@@ -170,7 +168,6 @@ var ProviderSet = wire.NewSet(
 	admin.NewChannelHandler,
 	admin.NewChannelMonitorHandler,
 	admin.NewChannelMonitorRequestTemplateHandler,
-	admin.NewAvailableChannelHandler,
 	admin.NewPaymentHandler,
 
 	// AdminHandlers and Handlers constructors