refactor: move memorize feature to /decks/[deck_id]/learn route

- Delete (features)/memorize directory
- Create /decks/[deck_id]/learn with Memorize component and page
- Update InDeck.tsx to navigate to new learn route
- Fix homepage memorize link to point to /decks

src/app/page.tsx

@@ -73,7 +73,7 @@ export default async function HomePage() {
           color="#dd7486"
         ></LinkArea>
         <LinkArea
-          href="/memorize"
+          href="/decks"
           name={t("memorize.name")}
           description={t("memorize.description")}
           color="#cc9988"