♿️(frontend) expose loading state to assistive technology

Surface the page's loading state to assistive technology, so screen
readers can announce that the page is still loading instead of
reading a partially rendered state as if it were complete.

src/frontend/src/components/LoadingScreen.tsx

@@ -19,7 +19,9 @@ export const LoadingScreen = ({
       <Screen layout={layout} header={header} footer={footer}>
         <CenteredContent>
           <Center>
-            <p>{t('loading')}</p>
+            <p role="status" aria-live="polite">
+              {t('loading')}
+            </p>
           </Center>
         </CenteredContent>
       </Screen>