Monitor: 7 and 30 day views, traffic used, merged tiles

- ranges 7d (30 min step) and 30d (2 h step); Prometheus keeps 31d
- rate() window is at least the step, so long views show averages
  instead of a 2-minute sample; "now" values still use a 2m window
- overview returns used_down/used_up: increase() of the interface
  counters over the whole range
- tiles merged: traffic (with volume for the period), devices, load

internal/cli/monitor.go

@@ -29,7 +29,7 @@ const (
 	promDefaults    = "/etc/default/prometheus"
 	promMarker      = "# managed by forgetting"
 	promListen      = "127.0.0.1:9090"
-	promRetention   = "15d"
+	promRetention   = "31d" // с запасом на вид «30 дней» во вкладке «Монитор»
 )
 
 func mlog(s string) { fmt.Println("  " + s) }