New demo page with two independent BimEngine instances side by side,
draggable divider for resizing, and proper SDK-layer resize handling.
Replaces the iframe-based approach now that ManagerRegistry is instance-based.
Removes unused panelViewer vite config entry.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
