Duplicate _DOMAINS definition in config.py and sidebar.py #7

Problem

_DOMAINS is defined as a set in config.py:43 and as a list in sidebar.py:10 with the same values. Adding or changing a domain requires updating two files.

Solution

Keep _DOMAINS in config.py as the single source of truth. Import it in sidebar.py.

Acceptance Criteria

• Single _DOMAINS definition
• No behavior change

Difficulty: Easy