feat(paint-slots): create-in-place scene materials + paint-panel polish

Custom-create now pre-creates a scene material and opens its inline editor
in the build pane (no separate right-side PaintPanel, which is removed);
the brush + "Paint with" use a scene: ref so painting stores the ref and
edits propagate everywhere. Slot preview (shared + item) resolves scene
refs so hover shows the real material.

Material properties editor uses the shared SliderControl for roughness/
metalness/opacity; row action buttons use Tooltip instead of title; the
color input renders as a clean filled swatch.

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

packages/nodes/src/item/paint.ts

@@ -169,7 +169,14 @@ function buildPreviewMaterial(
   materialPreset: string | undefined,
 ): Material | null {
   const shading = useViewer.getState().shading
-  if (materialPreset) return createMaterialFromPresetRef(materialPreset, shading)
+  if (materialPreset) {
+    const parsed = parseMaterialRef(materialPreset)
+    if (parsed?.kind === 'scene') {
+      const sceneMaterial = useScene.getState().materials[parsed.id as SceneMaterialId]
+      return sceneMaterial ? createMaterial(sceneMaterial.material, shading) : null
+    }
+    return createMaterialFromPresetRef(materialPreset, shading)
+  }
   if (material) return createMaterial(material, shading)
   return null
 }