build(admin): drop the styles of the site from the panel
The panel still compiled the partials of the public theme: posts, forum sections, lists, user rows, albums, the homepage and their dark variants. The redesigned templates use none of them. They are removed; what the panel did rely on moves into its own ui styles: the compact pagination of a phone and pagination-no-padding, the not-allowed cursor of a disabled button, and the tables and pictures CKEditor inserts, whose dark look stays in _dark.scss scoped to the editor.
The flag of the languages page uses the flag class of the panel (admin-flag, formerly admin-topbar__flag), and .form-group, gone from Bootstrap 5, gives way to spacing utilities on the sign-in form, the consent form and the CKEditor input. The styles of the panel go from 327 KB to 311 KB.