feat: improve Claude binary detection for active NVM environments

- Add support for NVM_BIN environment variable detection
- Prioritize currently active NVM environment over other installations
- Consolidate NVM-related logic in find_nvm_installations function
- Maintain backward compatibility with existing detection methods

This change helps users who manage Node.js versions with NVM by
ensuring Claudia detects the Claude binary from the currently
active Node.js environment, resolving issues where the wrong
Claude version might be selected despite having multiple NVM
installations available.