Terminal background: decide 24-bit color like tcell, and log why

The check only looked at COLORTERM, but tcell also draws 24-bit color for
terminals whose description has it (xterm-ghostty, xterm-kitty), so in
Ghostty on macOS without COLORTERM the background was never used. Use
tcell's rules, and always log which background is used and why.

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

internal/ui/terminal_test.go

@@ -2,6 +2,7 @@ package ui
 
 import (
 	"image/color"
+	"strings"
 	"testing"
 
 	"github.com/gdamore/tcell/v2"
@@ -31,6 +32,11 @@ func TestParseTerminalAnswers(t *testing.T) {
 }
 
 func TestUseTerminalBackground(t *testing.T) {
+	// tcell's terminal database also reads these from the real environment
+	// (and keeps what it added to an entry because of them, so the test uses
+	// a terminal nothing else looks up).
+	t.Setenv("COLORTERM", "")
+	t.Setenv("TCELL_TRUECOLOR", "")
 	defer func(old tcell.Color) { tview.Styles.PrimitiveBackgroundColor = old }(tview.Styles.PrimitiveBackgroundColor)
 	env := func(m map[string]string) func(string) string { return func(k string) string { return m[k] } }
 	dark := terminalInfo{bg: color.RGBA{30, 30, 46, 255}, hasBG: true}
@@ -44,13 +50,16 @@ func TestUseTerminalBackground(t *testing.T) {
 		want bool
 	}{
 		{"dark background, true color", dark, truecolor, true},
-		{"no true color", dark, env(map[string]string{"TERM": "xterm-256color"}), false},
+		{"no true color", dark, env(map[string]string{"TERM": "vt100"}), false},
+		{"ghostty without COLORTERM", dark, env(map[string]string{"TERM": "xterm-ghostty"}), true},
+		{"true color disabled for tcell", dark, env(map[string]string{"TERM": "xterm-ghostty", "TCELL_TRUECOLOR": "disable"}), false},
 		{"light background keeps the default", light, truecolor, false},
 		{"no answer", terminalInfo{}, truecolor, false},
 	} {
 		tview.Styles.PrimitiveBackgroundColor = tcell.ColorBlack
-		if got := useTerminalBackground(tc.info, tc.env); got != tc.want {
-			t.Errorf("%s: %v", tc.name, got)
+		note := useTerminalBackground(tc.info, tc.env)
+		if got := strings.Contains(note, "own color"); got != tc.want {
+			t.Errorf("%s: %s", tc.name, note)
 		}
 		want := tcell.ColorBlack
 		if tc.want {