fix(tui): stable monitor count and universal group icons

Site count in tab label and footer now reflects total monitors
(excluding groups) regardless of collapse state. Down count also
computed from all sites so collapsed groups with down children
still surface in the badge. Replaced Nerd Font folder glyphs
with standard Unicode triangles for cross-font compatibility.

internal/tui/tab_sites.go

@@ -29,9 +29,9 @@ func typeIcon(siteType string, collapsed bool) string {
 		return "◆"
 	case "group":
 		if collapsed {
-			return ""
+			return "▶"
 		}
-		return ""
+		return "▼"
 	default:
 		return "·"
 	}