Adds a "New blank channel" action next to "Add Channel" on ChannelsScreen
(web/src/App.tsx) that POSTs CreateChannelRequest with Blazor's add-mode
defaults (ChannelEditor.razor's else branch) via the new createChannel client,
then navigates to the channel's editor. Distinct from "Add Channel", which
remains the library-to-lineup ChannelBuilder flow and is untouched.