feat(pos): PWA con Service Worker + sync offline

- Add manifest.json, sw.js with cache-first/network-first strategies
- Add sync-engine.js with IndexedDB queue for offline operations
- Add /pos/api/sync/inventory endpoint for offline inventory cache
- Add /pos/sw.js route for proper SW scope registration
- Generate PWA icons (192x192, 512x512)
- Update all 10 HTML templates with manifest link, theme-color meta,
  SW registration, and sync-engine script

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/templates/invoicing.html

@@ -6,6 +6,8 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0" />
   <title>Facturación CFDI — Nexus Autoparts POS</title>
   <link rel="stylesheet" href="/pos/static/css/tokens.css" />
+  <link rel="manifest" href="/pos/static/pwa/manifest.json" />
+  <meta name="theme-color" content="#F5A623" />
 
   <style>
     /* =========================================================================
@@ -2724,6 +2726,8 @@
   <script src="/pos/static/js/app-init.js"></script>
   <script src="/pos/static/js/sidebar.js"></script>
   <script src="/pos/static/js/invoicing.js"></script>
+  <script src="/pos/static/js/sync-engine.js"></script>
+  <script>if('serviceWorker' in navigator){navigator.serviceWorker.register('/pos/sw.js',{scope:'/pos/'});}</script>
 
 </body>
 </html>