Use 🛑 (red octagonal stop sign) for ERROR prefix

❌ is a thin black-and-white cross that gets lost against terminal
backgrounds and the ANSI red text. 🛑 is a solid red octagon that
reads unmistakably as 'stop/error' at a glance, even when the user
isn't reading the line carefully.

internal/ui/ui_test.go

@@ -23,7 +23,7 @@ func TestMessageMethodsPlain(t *testing.T) {
 		{"Info", func(w *Writer) { w.Info("status") }, "》 status\n"},
 		{"Notice", func(w *Writer) { w.Notice("note") }, "》 note\n"},
 		{"Warning", func(w *Writer) { w.Warning("oops") }, "⚠️  Warning: oops\n"},
-		{"Error", func(w *Writer) { w.Error("boom") }, "❌ ERROR: boom\n"},
+		{"Error", func(w *Writer) { w.Error("boom") }, "🛑 ERROR: boom\n"},
 		{"Progress", func(w *Writer) { w.Progress("p") }, "  》 p\n"},
 		{"Banner", func(w *Writer) { w.Banner("hello") }, "hello\n"},
 	}