[FIX] open_workshop: ensure ProductScreen context for new POS orders

When adding a new order via the customer sidebar, the ProductScreen
was not explicitly activated. As a result, the screen_data context
expected by pos_time_based_products was missing, and no play button
was shown for time-based products.

This patch explicitly calls `this.pos.showScreen("ProductScreen")`
after creating a new order to restore compatibility with third-party
POS components that rely on the current screen state.

static/src/js/ows_pos_customer_sidebar.js

@@ -18,6 +18,7 @@ export class OwsPosCustomerSidebar extends Component {
 
     addOrder() {
         this.pos.add_new_order();  // neue Order wird aktive Order
+        this.pos.showScreen("ProductScreen");
         this.pos.selectPartner();
         this.env.bus.trigger('partner-changed');  // ✅ korrektes Event feuern
     }