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/main_test.go

@@ -6,8 +6,8 @@ import (
 )
 
 // TestMain keeps the tests from asking the real terminal about graphics
-// support (the probe writes to and reads from /dev/tty).
+// support and its colors (the probe writes to and reads from /dev/tty).
 func TestMain(m *testing.M) {
-	kittyGraphicsSupported = func() bool { return false }
+	terminalAnswers = func() terminalInfo { return terminalInfo{} }
 	os.Exit(m.Run())
 }