fix: return to mobile app after web sign-in

[wgqqqqq]

Summary

• Preserve and validate the native app callback marker on the auth web client.
• Attempt openbitfun://auth/callback from the completion page with a manual fallback button.
• Validate Android and iOS callback URIs before waking the existing account poller.
• Keep regular mobile-web and desktop login flows unchanged.

Verification

• pnpm --filter openbitfun-miniapp-market-web type-check
• pnpm --filter openbitfun-miniapp-market-web test -- --run src/AccountSignIn.test.tsx
• pnpm --filter openbitfun-miniapp-market-web build
• :app:assembleDebug with Android Studio JDK and SDK

Android APK was installed on emulator-5554; the deep-link intent resolved to MainActivity.

Deployment note

This requires publishing the updated MiniApp market web container for the public auth page. Relay deployment is not required.