tui: Chat view footer rewrite

Rewrote footer handling to better handle truncation, and use
`ActiveModel` to return the (stylized) active model

pkg/tui/util/util.go

@@ -54,6 +54,13 @@ func Height(str string) int {
 	return strings.Count(str, "\n") + 1
 }
 
+func Width(str string) int {
+	if str == "" {
+		return 0
+	}
+	return ansi.PrintableRuneWidth(str)
+}
+
 // truncate a string until its rendered cell width + the provided tail fits
 // within the given width
 func TruncateToCellWidth(str string, width int, tail string) string {