[FEATURE] Add dark web .onion link checker

[NovaCode37]

Add a module that checks if a domain or organization has known
.onion (Tor hidden service) mirrors or dark web presence.

Scope:

• New module: modules/onion_checker.py
• Check known .onion directories (Ahmia, DarkSearch API)
• Integrate into domain scan pipeline
• Display results in ScanResults.tsx

References:

• Ahmia search API: https://ahmia.fi/api/
• DarkSearch API: https://darksearch.io/apidoc