Add localization support.

Right now, only two languages are supported, and only a fraction of strings
are already translated. There's also quite a bunch of todos left where
strings are assembled in the Model, which has no access to localization
functions. But that's something for iterative refinement over the next
months, and (especially when it comes to adding languages and translation
strings to templates) merge requests.

Squashed commit of the following:

commit 67d756f3
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 19:53:56 2025 +0200

    more translations

commit 8cb0d65e
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 18:54:12 2025 +0200

    sme more translations

commit ff12f010
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 18:53:31 2025 +0200

    Add language selection to account page

commit 9bf27132
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 16:42:28 2025 +0200

    Translate footer components

commit 90c2c650
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 16:16:50 2025 +0200

    Use Accept-Language header if user has no preferred languages

commit 814cb4a4
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 16:11:19 2025 +0200

    Add list of preferred languages to user settings

commit 731b7898
Author: Birte Kristina Friesel <derf@finalrewind.org>
Date:   Fri Jul 18 15:33:42 2025 +0200

    Localization with Locale::Maketext

    WiP, no suitable foundation for merge requests yet.

    Still todo:
    * override Accept-Language header via account settings
    * Adjust all the templates and frontend javascript

    Related to #223