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/sw.js

@@ -0,0 +1,134 @@
+// /home/Autopartes/pos/static/pwa/sw.js
+// Nexus POS — Service Worker v1
+
+const CACHE_NAME = 'nexus-pos-v1';
+
+const APP_SHELL = [
+  '/pos/login',
+  '/pos/sale',
+  '/pos/catalog',
+  '/pos/inventory',
+  '/pos/customers',
+  '/pos/invoicing',
+  '/pos/accounting',
+  '/pos/dashboard',
+  '/pos/config',
+  '/pos/reports',
+  '/pos/static/css/tokens.css',
+  '/pos/static/css/common.css',
+  '/pos/static/js/app-init.js',
+  '/pos/static/js/sidebar.js',
+  '/pos/static/js/login.js',
+  '/pos/static/js/pos.js',
+  '/pos/static/js/catalog.js',
+  '/pos/static/js/inventory.js',
+  '/pos/static/js/customers.js',
+  '/pos/static/js/invoicing.js',
+  '/pos/static/js/accounting.js',
+  '/pos/static/js/dashboard.js',
+  '/pos/static/js/config.js',
+  '/pos/static/js/reports.js',
+  '/pos/static/js/offline-banner.js',
+  '/pos/static/js/sync-engine.js',
+  '/pos/static/pwa/manifest.json',
+  '/pos/static/pwa/icon-192.png',
+  '/pos/static/pwa/icon-512.png'
+];
+
+// ─── Install: pre-cache app shell ────────────────────────────────
+self.addEventListener('install', function (event) {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then(function (cache) {
+      return cache.addAll(APP_SHELL);
+    }).then(function () {
+      return self.skipWaiting();
+    })
+  );
+});
+
+// ─── Activate: purge old caches ──────────────────────────────────
+self.addEventListener('activate', function (event) {
+  event.waitUntil(
+    caches.keys().then(function (names) {
+      return Promise.all(
+        names.filter(function (n) { return n !== CACHE_NAME; })
+             .map(function (n) { return caches.delete(n); })
+      );
+    }).then(function () {
+      return self.clients.claim();
+    })
+  );
+});
+
+// ─── Fetch strategy ──────────────────────────────────────────────
+self.addEventListener('fetch', function (event) {
+  var url = new URL(event.request.url);
+
+  // API calls → network-first
+  if (url.pathname.indexOf('/pos/api/') !== -1) {
+    event.respondWith(networkFirst(event.request));
+    return;
+  }
+
+  // Everything else → cache-first
+  event.respondWith(cacheFirst(event.request));
+});
+
+function cacheFirst(request) {
+  return caches.match(request).then(function (cached) {
+    if (cached) {
+      // Update cache in background
+      fetch(request).then(function (response) {
+        if (response && response.status === 200) {
+          caches.open(CACHE_NAME).then(function (cache) {
+            cache.put(request, response);
+          });
+        }
+      }).catch(function () { /* offline, ignore */ });
+      return cached;
+    }
+    return fetch(request).then(function (response) {
+      if (response && response.status === 200) {
+        var clone = response.clone();
+        caches.open(CACHE_NAME).then(function (cache) {
+          cache.put(request, clone);
+        });
+      }
+      return response;
+    });
+  });
+}
+
+function networkFirst(request) {
+  return fetch(request).then(function (response) {
+    if (response && response.status === 200) {
+      var clone = response.clone();
+      caches.open(CACHE_NAME).then(function (cache) {
+        cache.put(request, clone);
+      });
+    }
+    return response;
+  }).catch(function () {
+    return caches.match(request);
+  });
+}
+
+// ─── Background Sync ─────────────────────────────────────────────
+self.addEventListener('sync', function (event) {
+  if (event.tag === 'nexus-pos-sync') {
+    event.respondWith && event.waitUntil(
+      self.clients.matchAll().then(function (clients) {
+        clients.forEach(function (client) {
+          client.postMessage({ type: 'SYNC_REQUESTED' });
+        });
+      })
+    );
+  }
+});
+
+// ─── Message handler ─────────────────────────────────────────────
+self.addEventListener('message', function (event) {
+  if (event.data && event.data.type === 'SKIP_WAITING') {
+    self.skipWaiting();
+  }
+});