feat: prominent ML shipping config error message + skip-validation checkbox

- Add skip-validation checkbox for accounts where ML validation fails due to config
- Detect 'User has not mode' errors and show detailed actionable help box
- Include direct links to ML seller config and support

pos/templates/inventory.html

@@ -882,6 +882,10 @@
             <label>Costo de Envío (MXN)</label>
             <input type="number" id="meliShippingCost" value="150" min="0" step="1" />
           </div>
+          <div class="inv-field" style="display:flex;align-items:center;gap:8px;margin-top:var(--space-2);">
+            <input type="checkbox" id="meliSkipValidate" style="width:auto;height:auto;" />
+            <label for="meliSkipValidate" style="margin:0;font-size:var(--text-caption);color:var(--color-text-muted);text-transform:none;">Saltear validación y publicar directamente (si la validación falla por config de cuenta)</label>
+          </div>
         </div>
 
         <!-- Dynamic required attributes section -->
@@ -914,7 +918,7 @@
   <script src="/pos/static/js/pos-utils.js" defer></script>
   <script src="/pos/static/js/sidebar.js" defer></script>
   <script src="/pos/static/js/virtual-scroll.js?v=2" defer></script>
-  <script src="/pos/static/js/inventory.js?v=14" defer></script>
+  <script src="/pos/static/js/inventory.js?v=15" defer></script>
   <script src="/pos/static/js/offline-banner.js" defer></script>
   <script src="/pos/static/js/sync-engine.js" defer></script>
   <script>if('serviceWorker' in navigator){navigator.serviceWorker.register('/pos/sw.js',{scope:'/pos/'});}</script>