Fix logged-out web UI on smaller screens (#19263)

...javascript/mastodon/components/not_signed_in_indicator.js

app/javascript/mastodon/features/about/index.js

app/javascript/mastodon/features/compose/index.js

app/javascript/mastodon/features/explore/index.js

app/javascript/mastodon/features/getting_started/index.js

app/javascript/mastodon/features/home_timeline/index.js

app/javascript/mastodon/features/keyboard_shortcuts/index.js

app/javascript/mastodon/features/notifications/index.js

...avascript/mastodon/features/ui/components/columns_area.js

...vascript/mastodon/features/ui/components/compose_panel.js

app/javascript/mastodon/features/ui/components/header.js

...javascript/mastodon/features/ui/components/link_footer.js

...cript/mastodon/features/ui/components/navigation_panel.js

app/javascript/mastodon/features/ui/components/tabs_bar.js

app/javascript/mastodon/features/ui/index.js

app/javascript/mastodon/features/ui/util/async-components.js

app/javascript/mastodon/locales/en.json

app/javascript/styles/mastodon/components.scss

app/javascript/styles/mastodon/variables.scss