feat(frontend): visual QA pass — unified page titles, CTAs, native controls, 404 fixes

- add PageTitle and apply it on /download, /themes, /configs, /showcase, /link, 404
- reset range-slider thumbs in the theme editor; fold color picker into the swatch
- hide horizontal scrollbars in the ClickGui mock; clip html to kill 100vw pan
- pin the footer under short pages via flex AppShell; split 404 art from caption
- one landing CTA geometry (lv-cta-lg / quiet); CLOUD eyebrow; auth caption scrim
- showcase error hides sort/pager; query retry limited to 1

frontend/src/app/App.tsx

@@ -4,7 +4,15 @@ import { SessionProvider } from '../features/auth/session'
 import { ActiveThemeProvider } from '../features/themes/useActiveTheme'
 import { routes } from './routes'
 
-const queryClient = new QueryClient({ defaultOptions: { queries: { staleTime: 30_000 } } })
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      staleTime: 30_000,
+      // One retry only: a dead backend should show its error state fast, not spam the console.
+      retry: 1,
+    },
+  },
+})
 const router = createBrowserRouter(routes)
 
 export default function App() {