feat(ui): floating back-to-top button on long scrollable pages

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.

frontend/src/App.tsx

@@ -29,6 +29,7 @@ import NavSidebar from "./components/NavSidebar";
 import Sidebar from "./components/Sidebar";
 import Feed from "./components/Feed";
 import ChannelPage from "./components/ChannelPage";
+import BackToTop from "./components/BackToTop";
 import Channels, { type ChannelStatusFilter, type ChannelsView } from "./components/Channels";
 import Playlists from "./components/Playlists";
 import Stats from "./components/Stats";
@@ -667,6 +668,8 @@ export default function App() {
         <ReleaseNotes onClose={() => setNotesOpen(false)} highlight={notesHighlight} />
       )}
       <ErrorDialog />
+      {/* Re-binds to the active page's scroll container on navigation (page or channel change). */}
+      <BackToTop dep={channelView ? `chan:${channelView.id}` : page} />
     </div>
   );
 }