feat: add password authentication and SSH key passphrase support

[inureyes]

Summary

• Add password-based authentication support with -P/--password flag
• Automatically detect encrypted SSH keys and prompt for passphrases
• Fix default username to use current user instead of hardcoded 'root'

Key Changes

Authentication Enhancements

• Password authentication: New -P/--password flag enables secure password prompting
• Encrypted key support: Automatically detects encrypted SSH keys (both explicit and default paths) and prompts for passphrase
• Multiple key formats: Checks for ed25519, RSA, ECDSA, and DSA keys in order of preference
• Secure prompting: Uses rpassword crate for secure password/passphrase input without echo

Default User Improvement

• Current user detection: Uses logged-in user as default instead of 'root'
• Multiple detection methods: Checks USER, USERNAME, LOGNAME env vars and system username
• Security improvement: Prevents accidental root connections for non-root users

Documentation Updates

• Updated --help output with new authentication options
• Added comprehensive Authentication section to README.md
• Updated man page with password and passphrase documentation
• Improved CLI help text to indicate passphrase support

Test Plan

• Test password authentication with -P flag
• Test encrypted SSH key detection and passphrase prompting
• Verify current user is used as default (not root)
• Test with various SSH key types (ed25519, RSA, etc.)
• Verify all commands work with new auth methods (exec, upload, download, ping)
• Unit tests for authentication method selection