chore: use the back component in layouts instead of a static link

[JannikStreek]

Further Notes

Currently our back navigation from admin back to brainstorming triggers a full reload. This could be improved by leveraging the back component, which is already available. However, it triggers a live reload and therefore the current strategy in the root layout to either display the logo or back button does not work anymore.

This does not work for live view navigation changes, as layout doesn't get the update:

<%= if admin_route(@conn) do %>