Use justify-content: flex-start; on Sverdle CSS to avoid compatibility issues

[Tal500]

This also eliminates the Vite warnings:
[vite:css] start value has mixed support, consider using flex-start instead

Using justify-content: flex-start; has here the same result as justify-content: start; (they are different only if the flex direction is opposite to the natural), and it has a wider compatibility (see caniuseit).

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