Migrate to React Flow 12

[n0spaces]

React Flow 12 just got released and has a lot of breaking changes. Some new features include improved styling, better development tools, more node hooks, and server-side rendering. SSR could potentially help with node layouting, since we could render the nodes to get their computed size before running the layout engine.

Upgrading is probably not an urgent matter, but if we decide this project would benefit from one of the new features, then it's probably better to upgrade sooner rather than later.

Migration guide is here: https://reactflow.dev/learn/troubleshooting/migrate-to-v12

Just skimming through the guide, it looks like node size/position properties will need to be changed, and a bunch of custom types may need to be updated. I'm not sure if this project is affected by item 4 in the list, which states that node/edge properties are now immutable.