feat(theme): add catppuccin themes with transparency and 256-color support

Add a theme package providing all four catppuccin flavors (mocha,
frappe, macchiato, latte) mapped to the xterm-256 palette. Themes are
selected with -theme or the CTOP_THEME environment variable; -i inverts
the flavor. Background slots use the terminal's native color for
transparency (opt out with -solid / CTOP_SOLID), and termbox is switched
to Output256 so colors render correctly. Replace hardcoded colors in the
compact and single views with ctheme portrait slots.

cwidgets/compact/gauge.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/bcicen/ctop/cwidgets"
 	"github.com/bcicen/ctop/models"
+	"github.com/bcicen/ctop/theme"
 
 	ui "github.com/gizak/termui"
 )
@@ -75,7 +76,7 @@ func (w *GaugeCol) Buffer() ui.Buffer {
 	// percentage value and low-contrast color for structure
 	if w.Percent < 5 {
 		w.Percent = 5
-		w.BarColor = ui.ColorBlack
+		w.BarColor = theme.Attr("crust")
 	}
 
 	return w.Gauge.Buffer()