Merge KeychainHelper functionality into KeychainManager to provide
a single, unified API for all keychain operations.
- Remove KeychainHelper.swift and KeychainHelperProtocol
- Add generic save/load/delete methods to KeychainManagerProtocol
- Update NostrIdentityBridge to use KeychainManagerProtocol
- Update FavoritesPersistenceService to use KeychainManagerProtocol
- Update PreviewKeychainManager with new methods
- Update MockKeychain and add MockKeychainHelper typealias for backwards compatibility
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
