refactor(paint-slots): share scene IBL as viewer SceneEnvironment

Replace the editor-only EditorEnvironment wrapper with a SceneEnvironment
component exported from @pascal-app/viewer, mounted as an opt-in <Viewer>
child (still not baked into the Viewer component). One source of truth the
editor and the community public viewer both inject; embed/thumbnail
surfaces simply don't mount it. Sunset preset at environmentIntensity 0.6.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/viewer/src/index.ts

@@ -18,6 +18,7 @@ export {
   DEFAULT_HOVER_STYLES,
   SSGI_PARAMS,
 } from './components/viewer/post-processing'
+export { SceneEnvironment } from './components/viewer/scene-environment'
 export { WalkthroughControls } from './components/viewer/walkthrough-controls'
 export { useAssetUrl } from './hooks/use-asset-url'
 export { useGLTFKTX2 } from './hooks/use-gltf-ktx2'