fix: Prevent leading zero when backspacing a decimal

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

calculator/calculator.app.js

@@ -349,6 +349,11 @@ function buttonPress(val) {
       }
       break;
     case 'B':
+      if (isDecimal) {
+        isDecimal = false;
+        displayOutput(currNumber);
+        break;
+      }
       if (currNumber != null) {
         currNumber = currNumber.toString();
         if (currNumber.length > 1) {