Phase 3 paint UI.
- Extract reusable MaterialPropertiesEditor (shared by the custom active-
paint editor and the scene-material editor).
- Curated Colors swatch row in the picker; catalog presets fork-on-tweak
by seeding a custom material from the entry's previewColor.
- Scene materials section in MaterialPaintPanel (shown once any exist):
list with swatch, inline rename, 'used by N parts', paint-with,
edit (live-propagates to every referencing part via the renderer's
sceneMaterials dep), duplicate, delete.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
