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.go

@@ -119,11 +119,11 @@ func (w *Writer) Warning(format string, args ...any) {
 	_, _ = fmt.Fprintln(w.out, prefix+fmt.Sprintf(format, args...))
 }
 
-// Error prints "❌ ERROR: " in red followed by the message. Goes to the
+// Error prints "🛑 ERROR: " in red followed by the message. Goes to the
 // same writer as everything else; callers that want stderr should
 // construct a separate Writer for it.
 func (w *Writer) Error(format string, args ...any) {
-	prefix := "❌ " + w.paint(ansiRed+ansiBold, "ERROR: ")
+	prefix := "🛑 " + w.paint(ansiRed+ansiBold, "ERROR: ")
 	_, _ = fmt.Fprintln(w.out, prefix+fmt.Sprintf(format, args...))
 }