Live page: streamed updates and a sliding chart

The server pushes each new step over server-sent events
(GET /api/v1/live/stream) the moment it is sampled, so updates no
longer arrive in uneven pairs. The chart slides left steadily between
steps instead of jumping, and the big numbers count to their new
value. Both stay still with reduced motion.

main_test.go

@@ -1308,6 +1308,8 @@ func TestSpeeds(t *testing.T) {
 		k.samples = []PeerSample{{PublicKey: pub, RxBytes: rx, TxBytes: tx}}
 		sp.sample(t0.Add(time.Duration(sec) * time.Second))
 	}
+	_, ch, cancel := sp.Subscribe()
+	defer cancel()
 	step(0, 1000, 1000)
 	if n := len(sp.Since(0)); n != 0 {
 		t.Fatalf("first sample made %d points, want 0", n)
@@ -1321,6 +1323,9 @@ func TestSpeeds(t *testing.T) {
 	if got, want := pts[0].Peers["p1"], [2]int64{8000, 1000}; got != want {
 		t.Fatalf("speed = %v, want %v (down, up in bit/s)", got, want)
 	}
+	if got := <-ch; got.T != pts[0].T {
+		t.Fatalf("subscriber got step %d, want %d", got.T, pts[0].T)
+	}
 	if _, ok := pts[1].Peers["p1"]; ok {
 		t.Fatal("a counter reset reported a speed")
 	}