feat(pos): add 5 quick improvements — dark mode, email quotes, barcode scan, returns, offline catalog

1. Auto dark mode: detect system prefers-color-scheme, auto-switch industrial/modern theme
2. Email quotation endpoint: POST /quotations/:id/email sends HTML email via SMTP
3. Camera barcode scanner: BarcodeDetector API with getUserMedia overlay in catalog
4. Returns with warranty: POST /returns endpoint with stock restoration and sale status tracking
5. Partial offline catalog: cache top 500 parts in IndexedDB, search when offline

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

pos/static/js/catalog.js

@@ -1005,6 +1005,20 @@
     }
 
     // ─── EXPOSE GLOBALS (for backward compat) ───
+    // ─── BARCODE CAMERA SCAN ───
+    function startBarcodeScan() {
+        if (!window.NexusNative) {
+            alert('El modulo de escaneo no esta cargado.');
+            return;
+        }
+        window.NexusNative.scanBarcode().then(function (code) {
+            if (code) {
+                searchInput.value = code;
+                runSearch(code);
+            }
+        });
+    }
+
     window.CatalogApp = {
         toggleCart: toggleCart,
         goToCheckout: goToCheckout,
@@ -1018,6 +1032,7 @@
         vsModelChanged: vsModelChanged,
         vsEngineChanged: vsEngineChanged,
         vsClear: vsClearAll,
+        startBarcodeScan: startBarcodeScan,
     };
 
     // ─── INIT ───