The "avatar + name + open-on-YouTube" cell and the @handle-or-/channel/<id>
URL were copy-pasted across the channel manager, the discovery tab, the
subscribe notice and the player. Extract a single ChannelLink component
(optional in-app onView, middle-click opens YouTube) and a channelYouTubeUrl
helper, and route all four through them. Removes the NameCell / DiscoveryNameCell
duplication (the latter introduced with the discovery tab).
