Refactor: one colorFade transition replaces internal.Fade

textInputFade becomes colorFade in theme.go and serves AvatarGroup,
Image retry, Tag and EditableText, which used internal.Fade. MyGo already
skips element transitions under ReduceMotion, so dropping the Motion
check keeps behaviour.

checkbox.go

@@ -48,7 +48,7 @@ func Checkbox(c *ui.Context, value *bool, label string, opts CheckboxOptions) *u
 		bg, border := checkboxColors(k, on || mixed, hover, pressed)
 		box := ui.Box(c).Size(checkboxSize, checkboxSize).Shrink(0).Radius(theme.ControlRadius).
 			Background(bg).Border(theme.BorderWidth, border)
-		box.Transition(textInputFade)
+		box.Transition(colorFade)
 		box.DrawOver(func(p *ui.Painter, r ui.Rect) {
 			switch {
 			case mixed: