You can save decks to Deck Memo.
And you can copy the deck of Deck Memo to Clash Royale app.
Copy the deck on app's deck page.
Enter text containing deck link, and push "Add" button.
Deck Memo data is associated with cookie.
So if you delete cookie your deck memo disappear.
You can save at most 50 decks.
The Deck Memo that are not displayed 20 days is automatically deleted.