v0.4.2: iconen gerepareerd, lucht-achtergrond (skybox) met toggle

- bootstrap-icons: @font-face correct ingebed (selector .bi::before was stuk)
- core/skybox.js + assets/skybox-data.js (lazy), knop Lucht in alle 3D-weergaven
- woning: met lucht aan raster en sandwichpanelen onzichtbaar

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

index.html

@@ -118,6 +118,7 @@
   <script src="js/core/camera-memory.js"></script>
   <script src="js/core/view-filter.js"></script>
   <script src="js/core/ui-kit.js"></script>
+  <script src="js/core/skybox.js"></script>
   <script src="js/vendor/jspdf.umd.min.js"></script>
   <script src="js/core/pdf-sheet.js"></script>
   <script src="js/wall/wall-model.js"></script>