Posters in macOS Terminal as plain colored cells

Terminal.app draws block elements from the font, with the halves off the
cell's middle, so quadrant posters came out as rows of shifted dots. The
new cover style "cells" uses one background color per cell and no glyph;
auto picks it when TERM_PROGRAM is Apple_Terminal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019reK4DUGYqDUjniWYJvwUW

internal/config/config_test.go

@@ -100,7 +100,7 @@ func TestShowCovers(t *testing.T) {
 
 func TestCoverStyleValidation(t *testing.T) {
 	base := `{"radarr":{"url":"http://r","apikey":"k"},"sonarr":{"url":"http://s","apikey":"k"},"sabnzbd":{"url":"http://a","apikey":"k"},"ui":{"cover_style":`
-	for _, ok := range []string{`"auto"`, `"quadrant"`, `"sextant"`, `"kitty"`, `""`} {
+	for _, ok := range []string{`"auto"`, `"quadrant"`, `"sextant"`, `"cells"`, `"kitty"`, `""`} {
 		if _, err := Parse([]byte(base + ok + `}}`)); err != nil {
 			t.Errorf("%s rejected: %v", ok, err)
 		}