client/web: add ServerMode to web.Server

Adds a new Mode to the web server, indicating the specific
scenario the constructed server is intended to be run in. Also
starts filling this from the cli/web and ipn/ipnlocal callers.

From cli/web this gets filled conditionally based on whether the
preview web client node cap is set. If not set, the existing
"legacy" client is served. If set, both a login/lobby and full
management client are started (in "login" and "manage" modes
respectively).

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

ipn/ipnlocal/web_client.go

@@ -52,6 +52,7 @@ func (b *LocalBackend) WebClientInit() (err error) {
 
 	b.logf("WebClientInit: initializing web ui")
 	if b.webClient.server, err = web.NewServer(web.ServerOpts{
+		Mode: web.ManageServerMode,
 		// TODO(sonia): allow passing back dev mode flag
 		LocalClient: b.webClient.lc,
 		Logf:        b.logf,