fix: Replace regex with manual thousands separator

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

calculator/calculator.app.js

@@ -195,7 +195,19 @@ function fixFloat(n) {
 
 function addSeparators(s) {
   var parts = s.split(".");
-  parts[0] = parts[0].replace(/\B(?=(\d{3})+(?!\d))/g, ",");
+  var intPart = parts[0];
+  var sign = "";
+  if (intPart[0] === "-") {
+    sign = "-";
+    intPart = intPart.slice(1);
+  }
+  var result = "";
+  while (intPart.length > 3) {
+    result = "," + intPart.slice(-3) + result;
+    intPart = intPart.slice(0, -3);
+  }
+  result = intPart + result;
+  parts[0] = sign + result;
   return parts.join(".");
 }