feat: MercadoLibre integration + inventory bulk publish + WhatsApp bridge fixes

- Add MercadoLibre OAuth, listings, orders, webhooks and category search
- New marketplace_external_bp.py, meli_service.py, marketplace_external_service.py
- New marketplace_external.html/js with ML management UI
- Inventory: bulk publish to ML with category autocomplete, listing type and shipping selectors
- Inventory: new .btn--meli styles, select/label CSS fixes
- WhatsApp bridge: rate limiting, 440/515/408 error handling, stale watchdog
- DB migration v3.4_meli_integration.sql for marketplace_listings, orders, sync_queue
- Add Celery tasks for ML sync and webhook processing
- Sidebar: MercadoLibre navigation link

pos/templates/pos.html

@@ -32,6 +32,10 @@
       <span id="statusClock"></span>
     </div>
     <div class="status-bar__right">
+      <a href="/pos/dashboard" class="btn btn--ghost btn--sm" id="backToSystemBtn" style="margin-right:8px;display:none;">
+        <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M3 9l9-7 9 7v11a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/><polyline points="9 22 9 12 15 12 15 22"/></svg>
+        Sistema
+      </a>
       <div class="status-bar__user" aria-label="Usuario activo">
         <div class="status-bar__user-avatar" aria-hidden="true">--</div>
         <span id="employeeName">Empleado</span>
@@ -203,7 +207,7 @@
           <button class="btn-secondary-action" onclick="POS.saveQuotation()" title="Cotizacion (F4)">Cotizar</button>
           <button class="btn-secondary-action" onclick="POS.showLastSale()" title="Ultima venta (F5)">Ult.Venta</button>
           <button class="btn-secondary-action" onclick="POS.showCutZModal()" title="Corte Z - Cerrar caja">Corte Z</button>
-          <button class="btn-secondary-action danger" id="btnCancelSale" title="Cancelar (Esc)">Cancelar</button>
+          <button class="btn-secondary-action danger" id="btnCancelSale" onclick="POS.openCancelModal()" title="Cancelar (Esc)">Cancelar</button>
         </div>
       </div>
     </aside>
@@ -233,14 +237,14 @@
       <span class="fkey-key">F6</span><span class="fkey-label">Cajon</span>
     </div>
     <div class="fkey-sep"></div>
-    <div class="fkey" title="Cantidad +/-">
+    <div class="fkey" onclick="POS.changeQuantity()" title="Cantidad +/-">
       <span class="fkey-key">+/-</span><span class="fkey-label">Cantidad</span>
     </div>
-    <div class="fkey" title="Descuento">
+    <div class="fkey" onclick="POS.applyDiscount()" title="Descuento">
       <span class="fkey-key">*</span><span class="fkey-label">Descuento</span>
     </div>
     <div class="fkey-sep"></div>
-    <div class="fkey" id="fkeyEsc" title="Cancelar">
+    <div class="fkey" id="fkeyEsc" onclick="POS.openCancelModal()" title="Cancelar">
       <span class="fkey-key">Esc</span><span class="fkey-label">Cancelar</span>
     </div>
   </div>
@@ -563,7 +567,7 @@
   <script src="/pos/static/js/app-init.js" defer></script>
   <script src="/pos/static/js/push.js" defer></script>
   <script src="/pos/static/js/printer.js" defer></script>
-  <script src="/pos/static/js/pos.js?v=4" defer></script>
+  <script src="/pos/static/js/pos.js?v=5" defer></script>
 
   <script>
     // Cancel sale button wiring
@@ -577,11 +581,7 @@
       closeCancelModal();
       // Clear cart via POS module
       if (typeof POS !== 'undefined') {
-        // Clear each item
-        const tbody = document.getElementById('cartBody');
-        while (tbody && tbody.firstChild) {
-          POS.removeFromCart(0);
-        }
+        POS.clearCart();
         POS.clearCustomer();
       }
     });
@@ -614,6 +614,20 @@
     }
     updateClock();
     setInterval(updateClock, 30000);
+
+    // Show "Back to System" button when NOT in kiosk mode
+    (function checkKiosk() {
+      var btn = document.getElementById('backToSystemBtn');
+      if (!btn) return;
+      function showIfNotKiosk() {
+        try {
+          var isKiosk = window.isKioskEnabled && window.isKioskEnabled();
+          btn.style.display = isKiosk ? 'none' : 'inline-flex';
+        } catch (e) { btn.style.display = 'inline-flex'; }
+      }
+      showIfNotKiosk();
+      setInterval(showIfNotKiosk, 2000);
+    })();
   </script>
 
   <script src="/pos/static/js/chat.js" defer></script>