fix(playground): fix diagnostics tooltip being behind buttons

[WarningImHack3r]

Spotted that on Paolo's TWIS today: diagnostics that are high enough and long enough are obstructed by the top buttons, which have a z-index.

This PR addresses that issue: it removes the z-index: 999 on the buttons and adds a z-index: 2 to both parts of the buttons’ hover tooltips — likely the reason the original z-index was used for.

Before	After

The button tooltips are preserved

I'm not sure if there are other reasons for this high z-index value beyond the tooltips; if so, please let me know and I'll add them to the fix as well.

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