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/static/pwa/manifest.json

@@ -0,0 +1,13 @@
+{
+  "name": "Nexus POS",
+  "short_name": "NexusPOS",
+  "description": "Sistema de Punto de Venta para Refaccionarias",
+  "start_url": "/pos/login",
+  "display": "standalone",
+  "background_color": "#0d0d0d",
+  "theme_color": "#F5A623",
+  "icons": [
+    {"src": "/pos/static/pwa/icon-192.png", "sizes": "192x192", "type": "image/png"},
+    {"src": "/pos/static/pwa/icon-512.png", "sizes": "512x512", "type": "image/png"}
+  ]
+}