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>

js/floor/floor.js

@@ -706,7 +706,8 @@ function mountFloorEditor(container, callbacks) {
   dirLight.castShadow = true;
   scene.add(dirLight);
 
-  scene.add(new THREE.GridHelper(10, 20, 0x444444, 0x222222));
+  const grid = new THREE.GridHelper(10, 20, 0x444444, 0x222222);
+  scene.add(grid);
 
   const floorMaterials = roofMode ? RoofModel.createMaterials() : FloorModel.createMaterials();
 
@@ -726,6 +727,11 @@ function mountFloorEditor(container, callbacks) {
     isolatieVisible = v.isolatie;
     update3DScene();
   });
+  // Lucht-achtergrond (core/skybox.js).
+  let skyOn = Skybox.isOn();
+  const applySky = () => { grid.visible = !skyOn; Skybox.apply(scene, skyOn, 0x1a1a1a, () => skyOn); };
+  Skybox.createToggle(document.getElementById('floor-toolbar-3d'), (on) => { skyOn = on; applySky(); });
+  applySky();
 
   function update3DScene() {
     if (floorGroup3D) scene.remove(floorGroup3D);