On-line at https://xtecna.github.io/lor-deck-checker/
This site is as simple as it gets. Choose one of the rules, mark if a deck must be a singleton, paste the code of your decks and check if they follow the rules.
If you're an organizer instead, you can mass check everyone's decks using the "Organizer" tab.
Thank you, AloRenato, for your enourmous help with the reponsiveness and layout! You're the man!