plait-board/drawnix ? reverse-engineered prompt

Build me an open source whiteboard app that feels simple to use but can do a lot in one place.

I want a canvas where people can make mind maps, flowcharts, and freehand sketches, and also drop in images and text. It should support undo and redo, copy and paste, zooming and scrolling on a huge canvas, autosave in the browser, and export to PNG or a saved JSON file. Please make it work nicely on mobile too, and include a light and dark theme.

If it makes sense, add handy shortcuts and a clean plugin style so the app can grow later. It would be great if it could turn Mermaid text into flowcharts and markdown into mind maps. Use the current docs or examples online if you need to, and keep the result polished enough that someone could start using it right away.