Leaving aside the background technical infrastrutture, that I don't want to comment here, I appreciate the clean UI.
Some suggestions:

• Add an optional procedure to verify the seed, display a prominent alert on the main view until the backup is verified;
• Manage the back button/gesture to go back from a section, otherwise you exit the web page;
• Add a real-time sats-fiat converter and let users input in fiat;
• Remove the bottom credits, the about section is enough ;)