Theme responsiveness is broken in all themes

[Donatas Glodenis]

It can be fixed partially by updating the metadata to Bootstrap 5 format:
on line 22 of file src/View/Metadata.php replace

$this->meta[] = ['viewport' => 'width=device-width, initial-scale=1'];

with

$this->meta[] = ['name' => 'viewport', 'content' => 'width=device-width, initial-scale=1'];

it is enough to fix the Auth theme, but the Admin theme needs some restructuring for narrow screens: it opens with expanded menu, the elements do not fit at the top and the sandwich button is misplaced. The left menu maybe is not the best idea for phone screens.

I may offer a PR for fixing the metadata, but that should perhaps be done together with redoing the UI, which I am not particularly good at.

[Donatas Glodenis]

PR #389 re-enabled mobile view, fixed the most glaring of the display issues.