Use the terminal's own background color

The panes used palette color 0, which color themes such as Catppuccin set
to a grey lighter than their background: everything looked washed out and
grey text almost disappeared (seen in Ghostty on macOS). The startup probe
now also asks for the background color (OSC 11), and with 24-bit color and
a dark background that color is used. The probe always runs at startup, so
a later switch to real images in the Config view never has to ask the
terminal while the screen owns it.

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

internal/ui/cover.go

@@ -15,7 +15,6 @@ import (
 	"os/exec"
 	"runtime"
 	"strings"
-	"sync"
 	"time"
 
 	"github.com/gdamore/tcell/v2"
@@ -198,17 +197,10 @@ func resolveCovers(setting string, env func(string) string) (blocks coverStyle,
 	return coverStyleFor(setting, env), false
 }
 
-var (
-	kittyProbeOnce   sync.Once
-	kittyProbeResult bool
-)
-
-// kittyGraphicsSupported asks the terminal once (see probeKittyGraphics) and
-// remembers the answer. It is a variable so tests can replace it.
-var kittyGraphicsSupported = func() bool {
-	kittyProbeOnce.Do(func() { kittyProbeResult = probeKittyGraphics(300 * time.Millisecond) })
-	return kittyProbeResult
-}
+// kittyGraphicsSupported reports whether the terminal confirmed the Kitty
+// graphics protocol when it was asked at startup (see probeTerminal). It is a
+// variable so tests can replace it.
+var kittyGraphicsSupported = func() bool { return terminalAnswers().graphics }
 
 // flatPixelError is the squared RGB distance per pixel below which a cell is
 // drawn flat: smooth areas and noise should not turn into patterns.