Bug fixes:
- Fix Zeller's formula for Julian calendar (separate branch without
century correction) and use rem_euclid for negative modulo safety
- Pass country code (cc=) to isdayoff.ru API in holiday plugin
- Sync clap --version with Cargo.toml (was hardcoded "1.0.0")
Tests:
- Rename integration_tests.rs to unit_tests.rs (they are unit tests)
- Fix race condition on env vars in plugin tests (Mutex + remove_var)
- Fix incorrect assertions (color tty detection, locale fallback)
- Add missing test cases: Julian/Gregorian Zeller, display date
parsing, Sunday offset, week numbers, edge cases (87 tests total)
- Remove brittle version-checking tests
