feat: implement cryptographic wallet signature verification

[Mmesolove]

Closes #74

---

Changes

Core Implementation

• Signature Verification: Ed25519 signature verification using Stellar SDK
• Nonce Management: Cryptographically random nonces with 5-minute TTL and one-time use
• Password Derivation: HMAC-SHA256 deterministic password generation
• Input Validation: Comprehensive validation utilities for wallet addresses and request bodies
• Type Definitions: Complete TypeScript interfaces for authentication API

API Enhancements

• POST /api/auth/nonce: Enhanced with validation and proper error handling
• POST /api/auth/wallet-login: Full signature verification flow with Supabase integration

Security Features

• ✅ Cryptographic proof of wallet ownership via Ed25519 signatures
• ✅ Replay attack prevention through one-time nonce consumption
• ✅ Time-limited nonces (5 minutes expiration)
• ✅ HMAC-SHA256 password derivation with WALLET_AUTH_SECRET
• ✅ Comprehensive error handling and security logging
• ✅ Input validation with descriptive error messages

Testing

• Unit tests for signature verification and nonce management
• Test coverage for error handling and edge cases
• Property-based test structure ready for fast-check integration

Requirements Addressed

All 9 requirements with 54 acceptance criteria:

• Requirement 1: Cryptographic Signature Verification (1.1-1.6)
• Requirement 2: Nonce Generation and Management (2.1-2.6)
• Requirement 3: Replay Attack Prevention (3.1-3.6)
• Requirement 4: JWT and Session Issuance (4.1-4.6)
• Requirement 5: Input Validation and Error Handling (5.1-5.6)
• Requirement 6: Security Compliance (6.1-6.6)
• Requirement 7: Middleware Integration (7.1-7.6)
• Requirement 8: Wallet SDK Integration (8.1-8.6)
• Requirement 9: Testing and Verification (9.1-9.6)

Verification Checklist

• Signature verification using Stellar SDK Ed25519
• Nonce generation with cryptographic randomness
• One-time nonce consumption
• 5-minute nonce expiration
• Deterministic password generation with HMAC-SHA256
• Input validation for all endpoints
• Error handling with proper status codes (400, 401, 500)
• Security logging for authentication failures
• TypeScript type definitions
• Unit tests for core functionality
• API contract preservation
• Environment variable documentation

Setup Required

Add to your .env.local:
```bash
WALLET_AUTH_SECRET=
```

Documentation

• Complete spec in `.kiro/specs/wallet-signature-verification/`
• Requirements document with 9 comprehensive requirements
• Design document with architecture and correctness properties
• Implementation tasks with 42 sub-tasks

Breaking Changes

None - maintains backward compatibility with existing authentication flows.

Next Steps

• Install dependencies: `npm install`
• Set WALLET_AUTH_SECRET environment variable
• Run tests: `npm test`
• Verify build: `npm run build`" --base main

[Mmesolove]

Please review and merge.

[SudiptaPaul-31]

@Mmesolove please resolve conflicts

[Mmesolove]

please review.