Add connection clone and credential cleanup

[ocnc]

Summary

• add a Clone action to the connection list context menu
• duplicate cloned connection settings and copy any saved password/private key into the clone's own Keychain accounts
• clean up only the deleted connection's Keychain credentials and add regression tests for clone/delete behavior

Testing

• xcodebuild test -scheme Spectty -project Spectty.xcodeproj -destination 'platform=iOS Simulator,name=iPhone 16,OS=18.5'