Added 9 unit tests for src/utils.rs to cover time/date formatting logic.
**Tests added**:
- ✅ format_timestamp_with_tz with positive offset (+03:00)
- ✅ format_timestamp_with_tz with negative offset (-05:00)
- ✅ format_timestamp_with_tz with zero offset (UTC)
- ✅ format_timestamp_with_tz midnight wrap (23:00 + 2h = 01:00)
- ✅ format_timestamp_with_tz invalid timezone (fallback to +03:00)
- ✅ get_day - extract day from timestamp
- ✅ get_day_grouping - messages on same day
- ✅ format_datetime - full date and time with MSK
- ✅ parse_timezone_offset via public API
**Coverage**:
- format_timestamp_with_tz() - all edge cases
- parse_timezone_offset() - tested indirectly (private fn)
- get_day() - day calculation and grouping
- format_datetime() - partial coverage
**Result**: 54 unit tests pass (was 45, +9 new)
Related: REFACTORING_ROADMAP.md P4.11
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
