feat: Replace percent button with Pi button, update logic and tests

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

calculator/test.js

@@ -330,7 +330,6 @@ const unaryOps = {
   'r': { name: 'sqrt', fn: Math.sqrt },
   's': { name: 'x^2', fn: (a) => a * a },
   'i': { name: '1/x', fn: (a) => 1 / a },
-  '%': { name: '%', fn: (a) => a / 100 },
 };
 
 for (const op in unaryOps) {
@@ -501,5 +500,22 @@ test('Negative zero handling', () => {
   checkDisplay(-0);
 });
 
+test('Pi button', () => {
+  press('p');
+  checkDisplay(Math.PI);
+  press('*2=');
+  checkDisplay(Math.PI * 2);
+});
+
+test('Pi replaces current number entry', () => {
+  press('123p');
+  checkDisplay(Math.PI);
+});
+
+test('Operation with Pi', () => {
+  press('5*p=');
+  checkDisplay(5 * Math.PI);
+});
+
 // Run all the defined tests
 runTests();