🧪 Add tests for sessionColor

[toreleon]

🎯 What: Added missing unit tests for sessionColor function from src/tui/lib/theme.ts.
📊 Coverage: Covered all branch scenarios including "morning", "afternoon", "atc", "lunch", and a fallback/muted case.
✨ Result: Improved test coverage and reliability for theme utility functions.

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PR created automatically by Jules for task 1798774706903563432 started by @toreleon

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