feat(#1281): Platform-aware download buttons

[kingalg]

Pull Request Checklist

• [x] I have linked this PR to a related GitHub issue.
• [ ] I have added a description of the change (and Github issue number, if any) to the root CHANGELOG.md. - this is a change to gh-pages and it will not be published in any of Quiet versions.

(Optional) Mobile checklist

Please ensure you completed the following checks if you did any changes to the mobile package:

• [ ] I have run e2e tests for mobile
• [ ] I have updated base screenshots for visual regression tests

For screenshots from all platform check - https://github.com/TryQuiet/quiet/issues/1281

[kingalg]

@adrastaea thank you for checking my first commit. Could you review it after changes? All design details are in #1281