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/app.py

@@ -157,6 +157,49 @@ def create_app():
         conn.close()
         return jsonify({'items': items, 'count': len(items)})
 
+    @app.route('/pos/api/sync/top-parts', methods=['GET'])
+    @_require_auth()
+    def sync_top_parts():
+        """Get top 500 most-sold parts for offline catalog cache."""
+        from tenant_db import get_tenant_conn
+        conn = get_tenant_conn(g.tenant_id)
+        cur = conn.cursor()
+        branch_id = g.branch_id
+
+        cur.execute("""
+            SELECT i.part_number, i.name, i.brand, i.price_1, i.tax_rate,
+                   i.category, COALESCE(s.stock, 0) AS stock,
+                   COALESCE(sv.total_sold, 0) AS total_sold
+            FROM inventory i
+            LEFT JOIN (
+                SELECT inventory_id, COALESCE(SUM(quantity), 0) AS stock
+                FROM inventory_operations GROUP BY inventory_id
+            ) s ON s.inventory_id = i.id
+            LEFT JOIN (
+                SELECT si.inventory_id, SUM(si.quantity) AS total_sold
+                FROM sale_items si
+                JOIN sales sa ON si.sale_id = sa.id
+                WHERE sa.status IN ('completed', 'partially_returned')
+                GROUP BY si.inventory_id
+            ) sv ON sv.inventory_id = i.id
+            WHERE i.is_active = true AND i.branch_id = %s
+            ORDER BY COALESCE(sv.total_sold, 0) DESC
+            LIMIT 500
+        """, [branch_id])
+
+        parts = []
+        for r in cur.fetchall():
+            parts.append({
+                'part_number': r[0], 'name': r[1], 'brand': r[2],
+                'price': float(r[3]) if r[3] else 0,
+                'tax_rate': float(r[4]) if r[4] else 0.16,
+                'category': r[5] or '',
+                'stock': r[6], 'total_sold': r[7]
+            })
+        cur.close()
+        conn.close()
+        return jsonify({'parts': parts, 'count': len(parts)})
+
     return app
 
 if __name__ == '__main__':