fix: Apply negation to current number when new number is pressed

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

calculator/calculator.app.js

@@ -537,7 +537,7 @@ function buttonPress(val) {
       hasPressedNumber = false;
       break;
     case 'N':
-      if (results != null) {
+      if (results != null && !hasPressedNumber) {
         displayOutput(results *= -1);
         prevExpression = "-(" + prevExpression + ")";
       } else {