auto cover style: use sextants when fontconfig has a font with the glyphs

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/ui/kitty_test.go

@@ -143,6 +143,8 @@ func TestPlaceholderCell(t *testing.T) {
 }
 
 func TestKittyTerminalDetection(t *testing.T) {
+	defer func(old func() bool) { sextantFontAvailable = old }(sextantFontAvailable)
+	sextantFontAvailable = func() bool { return false } // not what these cases are about
 	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
 	for _, tc := range []struct {
 		name string