task(E7): event elements polish - curtain (wavy polyline), spotlight (cone polygon with angle/color), barrier (chain circles), stage height attribute

frontend/src/services/seatingService.ts

@@ -359,6 +359,7 @@ export class SeatingService {
         stroke: '#1a2e3f',
         strokeWidth: 2,
         label: cfg.label,
+        height: cfg.height, // Bühnenhöhe in cm (Task E7: stage height attr)
       },
     };
   }