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/static/css/pos-glass.css

@@ -18,14 +18,22 @@ body {
    SIDEBAR — Glass treatment
    ========================================================================== */
 
+/* Prevent flash/stun while sidebar.js replaces static sidebar markup */
 .sidebar,
 .pos-sidebar {
+  opacity: 0;
+  transition: opacity 0.15s ease;
   background: var(--glass-bg-strong) !important;
   backdrop-filter: blur(20px);
   -webkit-backdrop-filter: blur(20px);
   border-right: 1px solid var(--glass-border) !important;
 }
 
+body.sidebar-ready .sidebar,
+body.sidebar-ready .pos-sidebar {
+  opacity: 1;
+}
+
 .sidebar__logo {
   position: relative;
 }