Add shopping cart system on event detail billetterie

- Each billet has +/- quantity buttons with max quantity enforcement
- Line total per billet updated in real-time
- Cart total and article count at the bottom
- Commander button disabled when cart is empty
- Full billet description displayed
- JS module cart.js with 10 tests covering all cases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/js/cart.test.js

@@ -0,0 +1,131 @@
+import { describe, it, expect, beforeEach } from 'vitest'
+import { initCart } from '../../assets/modules/cart.js'
+
+function createBilletterie(billets) {
+    let html = '<div id="billetterie">'
+
+    for (const b of billets) {
+        html += `
+            <div data-cart-item data-billet-id="${b.id}" data-price="${b.price}" data-max="${b.max}">
+                <button data-cart-minus></button>
+                <input data-cart-qty type="number" min="0" max="${b.max || 99}" value="0" readonly>
+                <button data-cart-plus></button>
+                <span data-cart-line-total></span>
+            </div>
+        `
+    }
+
+    html += '<span id="cart-total"></span><span id="cart-count"></span><button id="cart-checkout" disabled></button></div>'
+    document.body.innerHTML = html
+}
+
+describe('initCart', () => {
+    beforeEach(() => {
+        document.body.innerHTML = ''
+    })
+
+    it('does nothing without billetterie element', () => {
+        expect(() => initCart()).not.toThrow()
+    })
+
+    it('does nothing without total element', () => {
+        document.body.innerHTML = '<div id="billetterie"></div>'
+        expect(() => initCart()).not.toThrow()
+    })
+
+    it('initializes with zero totals', () => {
+        createBilletterie([{ id: 1, price: '10.00', max: 5 }])
+        initCart()
+
+        expect(document.getElementById('cart-total').textContent).toBe('0,00 \u20AC')
+        expect(document.getElementById('cart-count').textContent).toBe('0')
+        expect(document.getElementById('cart-checkout').disabled).toBe(true)
+    })
+
+    it('increments quantity on plus click', () => {
+        createBilletterie([{ id: 1, price: '15.00', max: 10 }])
+        initCart()
+
+        document.querySelector('[data-cart-plus]').click()
+
+        expect(document.querySelector('[data-cart-qty]').value).toBe('1')
+        expect(document.querySelector('[data-cart-line-total]').textContent).toBe('15,00 \u20AC')
+        expect(document.getElementById('cart-total').textContent).toBe('15,00 \u20AC')
+        expect(document.getElementById('cart-count').textContent).toBe('1')
+        expect(document.getElementById('cart-checkout').disabled).toBe(false)
+    })
+
+    it('decrements quantity on minus click', () => {
+        createBilletterie([{ id: 1, price: '10.00', max: 5 }])
+        initCart()
+
+        document.querySelector('[data-cart-plus]').click()
+        document.querySelector('[data-cart-plus]').click()
+        document.querySelector('[data-cart-minus]').click()
+
+        expect(document.querySelector('[data-cart-qty]').value).toBe('1')
+        expect(document.getElementById('cart-total').textContent).toBe('10,00 \u20AC')
+    })
+
+    it('does not decrement below zero', () => {
+        createBilletterie([{ id: 1, price: '10.00', max: 5 }])
+        initCart()
+
+        document.querySelector('[data-cart-minus]').click()
+
+        expect(document.querySelector('[data-cart-qty]').value).toBe('0')
+        expect(document.getElementById('cart-total').textContent).toBe('0,00 \u20AC')
+    })
+
+    it('respects max quantity', () => {
+        createBilletterie([{ id: 1, price: '10.00', max: 2 }])
+        initCart()
+
+        document.querySelector('[data-cart-plus]').click()
+        document.querySelector('[data-cart-plus]').click()
+        document.querySelector('[data-cart-plus]').click()
+
+        expect(document.querySelector('[data-cart-qty]').value).toBe('2')
+        expect(document.getElementById('cart-total').textContent).toBe('20,00 \u20AC')
+    })
+
+    it('allows unlimited when max is 0', () => {
+        createBilletterie([{ id: 1, price: '5.00', max: 0 }])
+        initCart()
+
+        for (let i = 0; i < 50; i++) {
+            document.querySelector('[data-cart-plus]').click()
+        }
+
+        expect(document.querySelector('[data-cart-qty]').value).toBe('50')
+        expect(document.getElementById('cart-total').textContent).toBe('250,00 \u20AC')
+    })
+
+    it('calculates total for multiple billets', () => {
+        createBilletterie([
+            { id: 1, price: '10.00', max: 5 },
+            { id: 2, price: '25.00', max: 3 },
+        ])
+        initCart()
+
+        const plusBtns = document.querySelectorAll('[data-cart-plus]')
+        plusBtns[0].click()
+        plusBtns[0].click()
+        plusBtns[1].click()
+
+        // 2 * 10 + 1 * 25 = 45
+        expect(document.getElementById('cart-total').textContent).toBe('45,00 \u20AC')
+        expect(document.getElementById('cart-count').textContent).toBe('3')
+    })
+
+    it('disables checkout when cart is empty again', () => {
+        createBilletterie([{ id: 1, price: '10.00', max: 5 }])
+        initCart()
+
+        document.querySelector('[data-cart-plus]').click()
+        expect(document.getElementById('cart-checkout').disabled).toBe(false)
+
+        document.querySelector('[data-cart-minus]').click()
+        expect(document.getElementById('cart-checkout').disabled).toBe(true)
+    })
+})