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>

assets/modules/cart.js

@@ -0,0 +1,64 @@
+function formatEur(value) {
+    return value.toFixed(2).replace('.', ',') + ' \u20AC'
+}
+
+export function initCart() {
+    const billetterie = document.getElementById('billetterie')
+    if (!billetterie) return
+
+    const items = billetterie.querySelectorAll('[data-cart-item]')
+    const totalEl = document.getElementById('cart-total')
+    const countEl = document.getElementById('cart-count')
+    const checkoutBtn = document.getElementById('cart-checkout')
+    if (!totalEl || !countEl) return
+
+    function updateTotals() {
+        let total = 0
+        let count = 0
+
+        for (const item of items) {
+            const price = Number.parseFloat(item.dataset.price) || 0
+            const qtyInput = item.querySelector('[data-cart-qty]')
+            const lineTotalEl = item.querySelector('[data-cart-line-total]')
+            const qty = Number.parseInt(qtyInput.value, 10) || 0
+
+            const lineTotal = price * qty
+            total += lineTotal
+            count += qty
+
+            lineTotalEl.textContent = formatEur(lineTotal)
+        }
+
+        totalEl.textContent = formatEur(total)
+        countEl.textContent = String(count)
+
+        if (checkoutBtn) {
+            checkoutBtn.disabled = count === 0
+        }
+    }
+
+    for (const item of items) {
+        const qtyInput = item.querySelector('[data-cart-qty]')
+        const minusBtn = item.querySelector('[data-cart-minus]')
+        const plusBtn = item.querySelector('[data-cart-plus]')
+        const max = Number.parseInt(item.dataset.max, 10) || 0
+
+        minusBtn.addEventListener('click', () => {
+            const current = Number.parseInt(qtyInput.value, 10) || 0
+            if (current > 0) {
+                qtyInput.value = current - 1
+                updateTotals()
+            }
+        })
+
+        plusBtn.addEventListener('click', () => {
+            const current = Number.parseInt(qtyInput.value, 10) || 0
+            if (max === 0 || current < max) {
+                qtyInput.value = current + 1
+                updateTotals()
+            }
+        })
+    }
+
+    updateTotals()
+}