[Frontend] Implement Secure Key Management and Wallet Recovery

[ONEONUORA]

🎯 Objective

Build secure key storage, backup, and recovery system for user wallets.

📋 Requirements

• Secure private key generation and storage
• 12/24-word mnemonic phrase generation
• Encrypted key storage using device keychain
• Backup and recovery flow
• Export private key (with warnings)
• Multi-account support
• Key rotation capability

🔧 Technical Details

• Use expo-secure-store or platform keychain
• Implement BIP39 mnemonic generation
• Encrypt keys with device-specific encryption
• Never send keys to backend
• Implement secure clipboard handling
• Add screenshot prevention for sensitive screens
• Use Stellar SDK for key derivation

✅ Acceptance Criteria

• Keys stored securely in device keychain
• Mnemonic phrase shown once during setup
• User can backup mnemonic phrase
• Recovery flow works correctly
• Keys never leave the device unencrypted
• Screenshot blocked on sensitive screens
• Clipboard cleared after copying keys
• Support for multiple accounts

Priority: Critical
Component: Frontend/Security