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/components/viewer/scene-environment.tsx

@@ -0,0 +1,22 @@
+'use client'
+
+import { Environment } from '@react-three/drei'
+import { Suspense } from 'react'
+
+/**
+ * Scene IBL — drei's prefiltered environment map, exported as an opt-in
+ * <Viewer> *child* rather than baked into the Viewer component, so embed /
+ * thumbnail surfaces that don't want the HDRI fetch simply don't mount it.
+ * This is what gives PBR metals their reflections and lifts the lighting on
+ * vertical surfaces (walls), which flat directional + hemisphere lights can't
+ * do alone. Intensity is dialled below the preset default so it complements
+ * the scene lights rather than washing them out. Only visible in `rendered`
+ * shading.
+ */
+export function SceneEnvironment() {
+  return (
+    <Suspense fallback={null}>
+      <Environment preset="sunset" environmentIntensity={0.6} />
+    </Suspense>
+  )
+}