Ask the terminal before using Kitty graphics; no fontconfig-based sextants on macOS

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

internal/ui/main_test.go

@@ -0,0 +1,13 @@
+package ui
+
+import (
+	"os"
+	"testing"
+)
+
+// TestMain keeps the tests from asking the real terminal about graphics
+// support (the probe writes to and reads from /dev/tty).
+func TestMain(m *testing.M) {
+	kittyGraphicsSupported = func() bool { return false }
+	os.Exit(m.Run())
+}