[Enhancement] Strengthen offline support positioning in the README

[sonukapoor]

Summary:
Improves the README again so offline advisory DB support is presented as a core differentiator, not just an additional feature.

Closes #82.

What changed:

• strengthened the hero and intro messaging around offline support
• added a short section explaining why offline mode matters
• clarified the distinction between online scan, advisory sync, and offline scan workflows
• strengthened enterprise and restricted-environment positioning
• expanded the comparison section with explicit offline support differentiation
• made offline support more prominent in the standout and use case sections

Notes:
This PR is documentation-only.

It focuses on adoption, differentiation, and making one of the project’s biggest recent wins much easier to understand quickly.

Verification:

• reviewed README content locally