feat(pos): add open-register UI flow

- Add 'Open Register' modal with register number and opening amount inputs
- loadRegister now shows clickable warning when no register is open
- checkout() opens register modal instead of plain alert when no register
- Add openRegister() API call to POST /pos/api/register/open
- Expose showOpenRegisterModal, closeOpenRegisterModal, openRegister globally
- Add cache-bust query params to pos.css and pos.js

pos/static/js/pos.js

@@ -120,14 +120,48 @@ const POS = (() => {
                 document.getElementById('registerInfo').innerHTML =
                     `<span>Caja #${data.register.register_number}</span>`;
             } else {
+                currentRegister = null;
                 document.getElementById('registerInfo').innerHTML =
-                    '<span>Sin caja abierta</span>';
+                    '<span style="color:var(--color-error);cursor:pointer;" onclick="POS.showOpenRegisterModal()" title="Clic para abrir caja">&#x26A0; Sin caja abierta — Clic para abrir</span>';
             }
         } catch (e) {
             console.warn('Register check failed:', e);
         }
     }
 
+    function showOpenRegisterModal() {
+        document.getElementById('openRegisterModal').classList.add('open');
+        document.getElementById('registerOpenResult').innerHTML = '';
+    }
+    function closeOpenRegisterModal() {
+        document.getElementById('openRegisterModal').classList.remove('open');
+        document.getElementById('registerOpenResult').innerHTML = '';
+    }
+    async function openRegister() {
+        const number = parseInt(document.getElementById('regNumber').value);
+        const amount = parseFloat(document.getElementById('regOpeningAmount').value) || 0;
+        if (!number || number < 1) {
+            document.getElementById('registerOpenResult').innerHTML = '<span style="color:var(--color-error);">Numero de caja invalido</span>';
+            return;
+        }
+        try {
+            const data = await api('/pos/api/register/open', {
+                method: 'POST',
+                body: JSON.stringify({ register_number: number, opening_amount: amount })
+            });
+            if (data.error) {
+                document.getElementById('registerOpenResult').innerHTML = '<span style="color:var(--color-error);">' + (data.error || 'Error') + '</span>';
+                return;
+            }
+            currentRegister = data;
+            document.getElementById('registerInfo').innerHTML = `<span>Caja #${data.register_number}</span>`;
+            closeOpenRegisterModal();
+            showToast(`Caja #${data.register_number} abierta con $${amount.toFixed(2)}`);
+        } catch (e) {
+            document.getElementById('registerOpenResult').innerHTML = '<span style="color:var(--color-error);">Error de red</span>';
+        }
+    }
+
     // ─── Cart ────────────────────────────
     function addToCart(item) {
         const existing = cart.find(c => c.inventory_id === item.inventory_id);
@@ -548,7 +582,7 @@ const POS = (() => {
     // ─── Payment ─────────────────────────
     function checkout() {
         if (cart.length === 0) { showToast('Carrito vacio'); return; }
-        if (!currentRegister) { alert('No hay caja abierta. Abra una caja primero.'); return; }
+        if (!currentRegister) { showOpenRegisterModal(); return; }
 
         paymentMethod = 'efectivo';
         const total = getTotal();
@@ -1160,5 +1194,6 @@ const POS = (() => {
         showLastSale, openDrawer,
         showTicket, closeTicketModal, printTicket,
         connectThermal, thermalPrint,
+        showOpenRegisterModal, closeOpenRegisterModal, openRegister,
     };
 })();