Refactor main.py entry point

[Chocapikk]

Cleaned up main.py:

• Removed redundant variable assignments, argparse already handles defaults and None
• Fixed dual-purpose variables (asn/port were used as both preset keys and actual values)
• Extracted resolve_asn() and resolve_port() to keep main() readable
• Replaced if/elif cascades with dict-based lookups
• Added proper type, default and choices to argparse arguments

[Chocapikk]

@NSM-Barii