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/app.go

@@ -102,7 +102,7 @@ type overlay struct {
 func New(cfg *config.Config) *App {
 	// Ask the terminal before the screen takes it over and before any widget
 	// copies the default background.
-	ownBackground := useTerminalBackground(terminalAnswers(), os.Getenv)
+	backgroundNote := useTerminalBackground(terminalAnswers(), os.Getenv)
 	a := &App{
 		app:    tview.NewApplication(),
 		tabs:   tview.NewTextView().SetDynamicColors(true),
@@ -139,9 +139,7 @@ func New(cfg *config.Config) *App {
 	if a.covers {
 		a.logs.add(levelInfo, coverNote)
 	}
-	if ownBackground {
-		a.logs.add(levelInfo, "Background: the terminal's own color")
-	}
+	a.logs.add(levelInfo, backgroundNote)
 	sabClient := sabnzbd.NewClient(cfg.SABnzbd.URL, cfg.SABnzbd.APIKey)
 	a.sab = sabClient
 	a.probes = []probe{