feat: image error placeholder + scroll-to-top on navigation

Image bubbles previously used the raw filename as alt text, so a failed
asset load exposed the file name in the broken-image box. Add a reusable
BubbleImage that renders an empty alt and falls back to a neutral
placeholder (ImageOff icon) on error; use it in the album, image, and
image-with-text bubbles, and drop the filename from their aria-labels.

Also add a global ScrollToTop that resets the window on route change so
desktop navigation matches mobile (e.g. clicking a category card no
longer lands at the bottom of the new page). Hash navigations are skipped
so #post-<id> deep-link scrolling still works.

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

src/App.tsx

@@ -10,6 +10,7 @@ import { CategoryPage } from "./pages/Category";
 import { OfficialRecommendationsPage } from "./pages/OfficialRecommendations";
 import { SearchPage } from "./pages/Search";
 import { PostRedirect } from "./pages/PostRedirect";
+import { ScrollToTop } from "./components/ScrollToTop";
 import { AboutPage } from "./pages/About";
 import Favorites from "./pages/Favorites";
 import { adminUiPrefix } from "./adminPaths";
@@ -29,6 +30,7 @@ export default function App() {
             <ImageLightboxProvider>
               <VideoPlayerProvider>
                 <BrowserRouter>
+                  <ScrollToTop />
                   <Routes>
                     <Route element={<PublicLayout />}>
                       <Route path="/" element={<Home />} />