Add allow_zero_price_sales toggle: config UI, POS enforcement and backend validation

pos/static/js/config.js

@@ -625,6 +625,7 @@ const Config = (() => {
             await saveVehicleCompatSource();
             await saveAllowedBrands();
             await saveModules();
+            await saveSalesSettings();
             await saveReceiptConfig();
             toast('Configuración guardada', 'ok');
         } catch (e) {
@@ -1015,6 +1016,37 @@ const Config = (() => {
         }
     }
 
+    async function loadSalesSettings() {
+        try {
+            var res = await fetch(API + '/sales-settings', { headers: headers() });
+            if (!res.ok) return;
+            var data = await res.json();
+            var cb = document.getElementById('cfg-allow-zero-price');
+            if (cb) cb.checked = data.allow_zero_price_sales !== false;
+        } catch (e) {
+            console.error('Config.loadSalesSettings:', e);
+        }
+    }
+
+    async function saveSalesSettings() {
+        var cb = document.getElementById('cfg-allow-zero-price');
+        if (!cb) return;
+        try {
+            var res = await fetch(API + '/sales-settings', {
+                method: 'PUT',
+                headers: headers(),
+                body: JSON.stringify({ allow_zero_price_sales: cb.checked })
+            });
+            if (!res.ok) {
+                var err = await res.json().catch(function() { return { error: res.statusText }; });
+                throw new Error(err.error || 'Save failed');
+            }
+        } catch (e) {
+            toast(e.message, 'error');
+            throw e;
+        }
+    }
+
     // -------------------------------------------------------------------------
     // Tab navigation
     // -------------------------------------------------------------------------
@@ -1277,6 +1309,7 @@ const Config = (() => {
         loadVehicleCompatSource();
         loadAllowedBrands();
         loadModules();
+        loadSalesSettings();
         loadReceiptConfig();
         if (isAdmin) {
             loadRolePermissions();