fix(site): give logo image a label and bump site-kit

[geoffrich]

Fixes a couple a11y issues on the SvelteKit site - see sveltejs/site-kit#107 for details on the site-kit a11y fixes

Fixes:

• SvelteKit logo on homepage was not properly labelled
• Dark mode toggle was incorrectly nested inside a <ul> (site-kit fix)
• Dark mode toggle was not labelled properly (site-kit fix)
• Mobile navigation did not have background set (site-kit fix)

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Latest commit: 1258d6e

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