feat: Add backspace button to keypad

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

calculator/calculator.app.js

@@ -28,9 +28,9 @@ var swipeEnabled;
 
 var numbersGrid = [3, 4];
 var numbers = {
-  '0': {grid: [1, 3], globalGrid: [1, 4], trbl: '2.00'},
+  '0': {grid: [1, 3], globalGrid: [1, 4], trbl: '2.0B'},
   '.': {grid: [2, 3], globalGrid: [2, 4], trbl: '3=.0'},
-  '1': {grid: [0, 2], globalGrid: [0, 3], trbl: '4201'},
+  '1': {grid: [0, 2], globalGrid: [0, 3], trbl: '42B1'},
   '2': {grid: [1, 2], globalGrid: [1, 3], trbl: '5301'},
   '3': {grid: [2, 2], globalGrid: [2, 3], trbl: '6+.2'},
   '4': {grid: [0, 1], globalGrid: [0, 2], trbl: '7514'},
@@ -39,6 +39,7 @@ var numbers = {
   '7': {grid: [0, 0], globalGrid: [0, 1], trbl: 'R847'},
   '8': {grid: [1, 0], globalGrid: [1, 1], trbl: 'N957'},
   '9': {grid: [2, 0], globalGrid: [2, 1], trbl: '%*68'},
+  'B': {grid: [0, 3], globalGrid: [0, 4], trbl: '10BB', val: '<-', color: COLORS.SPECIAL},
 };
 
 var operatorsGrid = [2, 3];
@@ -320,6 +321,22 @@ function buttonPress(val) {
         displayOutput(currNumber *= -1);
       }
       break;
+    case 'B':
+      if (currNumber != null) {
+        currNumber = currNumber.toString();
+        if (currNumber.length > 1) {
+          currNumber = currNumber.slice(0, -1);
+        } else {
+          currNumber = '0';
+        }
+        // if we removed a decimal point
+        if (currNumber.indexOf('.') === -1) {
+          isDecimal = false;
+        }
+        hasPressedNumber = currNumber;
+        displayOutput(currNumber);
+      }
+      break;
     case '/':
     case '*':
     case '-':