feat(backend): retire mock wallet auth and unify challenge verification

[Ezeh20]

Pull Request Template

Description

Retires the legacy mock wallet authentication path and makes the SEP-10 challenge/verify flow the canonical production auth model for both login and registration.

Fixes #306

This change removes legacy mock signature verification from production auth flows, centralizes secure wallet verification in a shared backend helper, preserves replay protection through used challenge tracking, and updates the wallet auth UI to sign server-issued Stellar challenges instead of plain messages.

---

Checklist

• I have tested my changes locally
• I have run npx prisma generate after schema changes
• I have run npx prisma migrate dev or npx prisma migrate deploy as appropriate

Notes:

• No Prisma schema changes were made in this PR, so no migration was required.
• Local verification run:
  • corepack pnpm exec vitest run tests/api/auth-credentials.test.ts tests/lib/wallet-auth.test.ts
• Prisma client generation run:
  • npx prisma generate

---

Post-Merge Steps for Maintainers

If this PR includes changes to the Prisma schema:

1. Run the following command to apply the migration to your database:

   npx prisma migrate deploy

[drips-wave]

@Ezeh20 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits