gs-details-page: Improve "Loading app details" status page design

Make the status page similar to the "Loading Updates" status page from gs-updates-page.ui, using the same elements spacing and arrangement (with AdwClamp), text size and text weight. Also, use header capitalization to follow HIG guidelines.

I basically copied the status page code from gs-updates-page: https://gitlab.gnome.org/GNOME/gnome-software/-/blob/gnome-45/src/gs-updates-page.ui?ref_type=heads#L21 and made the necessary adaptations.

Before	After	Loading Updates page

I also included !1792 (merged)