If `reactflow` initializes before the node templates are parsed, edges may not be rendered and the viewport may get reset.
- Add `isReady` state to `NodesState`. This is false when we are loading or parsing node templates and true when that is finished.
- Conditionally render `reactflow` based on `isReady`.
- Add `viewport` to `NodesState` & handlers to keep it synced. This allows `reactflow` to mount and unmount freely and not lose viewport.
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