Layout is now: fixed eraser/reset + category tabs (no longer sticky-inside-
scroll), a single scrolling catalog grid, and an always-visible scene-material
footer. The custom-material "+" moved out of the colors-only grid cell onto the
scene-material section header, so a custom material can be added from any
category (creates a blank scene material, selects it as the brush, opens its
inline editor).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
