furnishup/blueprint3d ? reverse-engineered prompt

Build me a web app for planning interior spaces in 2D and 3D, like a simple home or apartment designer. I want users to draw a floor plan, place walls and rooms, add furniture and other items, then switch into a 3D view to see the layout rendered nicely. It should feel like a polished demo app that works in the browser, with a clean editing experience and an example page I can open locally and try right away.

Please keep it flexible so the core design tools can be reused in other projects, and make sure the floor plan and item placements can be saved and loaded again. If you need to look up current docs for any library pieces, go ahead and do that. Also include a straightforward way to run and test it locally so I can see the editor working end to end.