feat: scroll to post bubble from recommended card + back-to-top button

Recommended cards already routed to /browse#post-<id>, but the stream had
no logic to scroll to the target bubble — and the post might not be paged
in yet. MessageStream now resolves the #post-<id> hash, auto-loads more
pages until the bubble renders, scrolls to it, and gives it a brief gold
highlight. Bubbles get scroll-mt so they clear the sticky header.

Also adds a global floating back-to-top button (BackToTop) mounted in
PublicLayout, shown after scrolling past 400px.

Bundles related staging UI work already present in the working tree.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -1,5 +1,7 @@
 import { BrowserRouter, Navigate, Route, Routes } from "react-router-dom";
 import { I18nProvider } from "./i18n";
+import { MotionProvider } from "./motion";
+import { ToastProvider } from "./components/Toast";
 import { PublicLayout } from "./layouts/PublicLayout";
 import { Home } from "./pages/Home";
 import { Browse } from "./pages/Browse";
@@ -21,7 +23,9 @@ const adminEnabled = import.meta.env.VITE_DISABLE_ADMIN !== "true";
 export default function App() {
   return (
     <I18nProvider>
-      <AdminRouterModeProvider value="absolute">
+      <MotionProvider>
+        <ToastProvider>
+        <AdminRouterModeProvider value="absolute">
         <ImageLightboxProvider>
           <VideoPlayerProvider>
             <BrowserRouter>
@@ -55,7 +59,9 @@ export default function App() {
             </BrowserRouter>
           </VideoPlayerProvider>
         </ImageLightboxProvider>
-      </AdminRouterModeProvider>
+        </AdminRouterModeProvider>
+        </ToastProvider>
+      </MotionProvider>
     </I18nProvider>
   );
 }