feat(mobile): implement wallet-based authentication service with Stel…

[NueloSE]

Summary

• Add WalletAuthService with full Stellar wallet authentication flow (challenge → sign → verify)
• Add src/services/auth/types.ts with AuthResponse, ChallengeResponse, LinkWalletResponse, AuthError, and AuthErrorCode types
• Integrate with existing StellarWalletService for message signing and expo-secure-store for JWT token persistence

Changes

• src/services/auth/types.ts — wallet auth types and typed error class
• src/services/auth/walletAuth.service.ts — WalletAuthService implementing getChallenge, authenticate, walletLogin, linkWallet, and unlinkWallet
• src/services/auth/__tests__/walletAuth.service.test.ts — 23 unit tests covering happy paths and error scenarios

API Endpoints Consumed

Endpoint	Method
/auth/wallet/challenge	POST
/auth/wallet/verify	POST
/auth/wallet/link	POST
/auth/wallet/unlink	DELETE

Test Plan

• All 23 unit tests pass (./node_modules/.bin/jest src/services/auth/__tests__/walletAuth.service.test.ts)

Closes #114

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[Oluwaseyi89]

@NueloSE please resolve the merge conflict. Contact me once done.