fix: ignore backspace when the stack is empty

.memory/state.md

@@ -5,5 +5,5 @@
 - Public API: `push`, `pop`, `clear`, `swap`, `remove`, `edit`, `isValidIndex`, `input`, `command`, `getOperationsByCategory`, `getConstants`
 - Config: `maxSize`, `base`, `angleMode`, `enabledCommands`
 - Commands: arithmetic, stack, trigonometry, constants `pi` and `e`
-- Demo actions: paste now parses clipboard text as a number before pushing it to the stack; Ctrl+V is supported via the hidden input paste event
+- Demo actions: paste now parses clipboard text as a number before pushing it to the stack; Ctrl+V is supported via the hidden input paste event; backspace is ignored when the stack is empty
 - Exports: browser `window.RpnCalculator`, CommonJS `module.exports`

samples/calc-02/index.js

@@ -162,9 +162,10 @@ function execute(name) {
 		} else if (name === 'backspace') {
 			if (calc.isEditing) {
 				inputToX('Backspace');
-			} else {
+			} else if (calc.isValidIndex(0)) {
 				calc.remove(0);
 			}
+
 		} else if (name === 'swap') {
 			pushEditingValueIfNeeded();
 			if (calc.isValidIndex(1)) {