Adds two floating bookshelves to the Nexus 3D scene, each holding
books whose spines display merged PR numbers and titles. Books are
built from canvas textures via the Gitea API, with fallback data if
unreachable. Both shelves gently bob in the scene.
Refs #264
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>