feat(mobileapp): handle Privy wallet signup linking

[Jerry-Tekh]

Summary

• resolve or create the authenticated user’s Privy embedded Ed25519 wallet without accessing private key material
• derive its Stellar address, sign the backend registration challenge through the Privy provider, and verify wallet ownership
• link the Privy DID through /api/auth/privy, cache the returned Zaps JWT only after success, and surface backend errors
• add focused tests for generated and existing wallets, signature encoding, route payloads, and rejected links

Testing

• npm test -- --runInBand (10 suites, 51 tests)
• npx eslint src/services/api.ts __tests__/api.test.ts --max-warnings 0
• npx prettier --check src/services/api.ts __tests__/api.test.ts
• scoped TypeScript check for src/services/api.ts

Closes #584

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[ONEONUORA]

Great job @Jerry-Tekh