feat: add ukraine support banner

[pngwn]

This PR adds a banner at the bottom of the site showing our support for Ukraine in this time of crisis. It also provides a link directing people to support the efforts. I use the colours of the flag of Ukraine, as best I could emulate them.

I have added some padding and tweaked some heights to ensure that content is not hidden by the banner (particularly on mobile). Screenshots below

[vercel]

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svelte-dev – ./sites/svelte.dev

🔍 Inspect: https://vercel.com/svelte/svelte-dev/6GQ6Au87r72eM2kVnHSkmfGoZ5dK
✅ Preview: https://svelte-dev-git-ukr-banner-svelte.vercel.app

hn – ./sites/hn.svelte.dev

🔍 Inspect: https://vercel.com/svelte/hn/Hdto4eSynRb2LGbrWKtQbQjjGXyC
✅ Preview: https://hn-git-ukr-banner-svelte.vercel.app

[kevmodrome]

ACK

[dummdidumm]

This looks good on mobile everywhere except the docs. On FF the banner is placed weirdly and Chrome is completely confused.
Side note: do we want to make this closable? Especially if it's causing layout issues like mentioned above it might be good for some, but I also understand the arguments for making it unclosable.

[pngwn]

@dummdidumm Do you have some screenshots? It looks okay for me on chrome. Will check FF.

I don't want to make it closable.

[pngwn]

@vedam created a nice svelte + Ukraine flag logo, should we add it? I like it.

[pngwn]

Pushed the version with the logo @vedam provided so you can see for yourself. Can remove it if needed.

[dummdidumm]

The banner stays at the bottom correctly now on mobile, but the banner hides the mobile menu that is in the bottom left.

[pngwn]

@dummdidumm Fixed in latest commit.