🐾 Minor Update: Add key property to LevelSelector divs

[TomBinford]

The LevelSelector component creates a list of divs by mapping over a dummy array. Because a key property is not present in the divs, React generates a warning. (I believe it's a performance concern.)

The fix is already implemented for the divs in the AxisSelector. Simply add a key property to the div created in the map lambda and set it to some unique value like i.

• 👋 I may be able to implement this update request

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This is a 🐾 Minor Update