A reusable BackToTop fades in once the active page's <main> is scrolled past ~600px and
smooth-scrolls it back to the top. Rendered once at the App root, portaled to <body> for
viewport-fixed placement (bottom-right, below the chat dock), and re-binds to the live <main> on
navigation (page or channel change) so it works on every scrollable page incl. the channel page.
EN/HU/DE.
