client/web: allow login client to still run tailscale up

I don't believe this has ever worked, since we didn't allow POST
requests in the login client. But previously, we were primarily using
the legacy client, so it didn't really matter. Now that we've removed
the legacy client, we have no way to login.

This fixes the login client, allowing it to login, but it still needs to
be refactored to expose a dedicated login method, without exposing all
the node update functionality.

Updates tailscale/corp#14335

Signed-off-by: Will Norris <will@tailscale.com>

client/web/web.go

@@ -349,6 +349,10 @@ func (s *Server) serveLoginAPI(w http.ResponseWriter, r *http.Request) {
 		// TODO(soniaappasamy): we may want a minimal node data response here
 		s.serveGetNodeData(w, r)
 		return
+	case httpm.POST:
+		// TODO(will): refactor to expose only a dedicated login method
+		s.servePostNodeUpdate(w, r)
+		return
 	}
 	http.Error(w, "invalid endpoint", http.StatusNotFound)
 	return