fix: cap undo history, fix btoa unicode crash, add quota fallback (#352)

[avinxshKD]

Fixes #352

1. actionArr was never pruned, capped at 100 entries, trims from the front
2. btoa() crashes on unicode node names , switched stringifyAction to plain JSON.stringify, parseAction falls back to atob for old saved data
3. localStorage quota exceeded silently killed the graph, save() now retries without actionHistory if the first write fails

[avinxshKD]

@pradeeban pls have a look

[pradeeban]

@avinxshKD merge conflicts. Pls check.