refactor: unify design-system components across pages

- Replace native textarea with Textarea in translator and text-speaker pages
- Replace custom loading spinners with Skeleton in InDeck and FavoritesClient pages
- Add shared constants DEFAULT_NEW_PER_DAY, DEFAULT_REV_PER_DAY

src/app/(features)/favorites/FavoritesClient.tsx

@@ -12,6 +12,8 @@ import { toast } from "sonner";
 import { PageLayout } from "@/components/ui/PageLayout";
 import { PageHeader } from "@/components/ui/PageHeader";
 import { CardList } from "@/components/ui/CardList";
+import { VStack } from "@/design-system/layout/stack";
+import { Skeleton } from "@/design-system/feedback/skeleton";
 import { actionGetUserFavoriteDecks, actionToggleDeckFavorite } from "@/modules/deck/deck-action";
 import type { ActionOutputUserFavoriteDeck } from "@/modules/deck/deck-action-dto";
 
@@ -102,10 +104,10 @@ export function FavoritesClient({ initialFavorites }: FavoritesClientProps) {
 
       <CardList>
         {loading ? (
-          <div className="p-8 text-center">
-            <div className="w-8 h-8 border-2 border-gray-200 border-t-gray-400 rounded-full animate-spin mx-auto mb-3"></div>
+          <VStack align="center" className="p-8">
+            <Skeleton variant="circular" className="w-8 h-8" />
             <p className="text-sm text-gray-500">{t("loading")}</p>
-          </div>
+          </VStack>
         ) : favorites.length === 0 ? (
           <div className="text-center py-12 text-gray-400">
             <div className="w-16 h-16 mx-auto mb-3 rounded-full bg-gray-100 flex items-center justify-center">