client/web: hook up auth flow

Connects serveTailscaleAuth to the localapi webclient endpoint
and pipes auth URLs and session cookies back to the browser to
redirect users from the frontend.

All behind debug flags for now.

Updates tailscale/corp#14335

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

ipn/localapi/localapi.go

@@ -2179,7 +2179,6 @@ func (h *Handler) serveDebugWebClient(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(resp.StatusCode)
 }
 
 func defBool(a string, def bool) bool {