refactor: remove unused qrCodeAtom and update mobile app prompts

Remove the unused `qrCodeAtom` from the QR code store to clean up the codebase. Additionally, update the mobile app development prompts to emphasize critical requirements, such as creating a default route, ensuring high-quality UI/UX, and following Expo best practices.

app/stores/qrCodeStore.ts

@@ -1,4 +1,3 @@
 import { atom } from 'nanostores';
 
-export const qrCodeAtom = atom<string | null>(null);
 export const expoUrlAtom = atom<string | null>(null);