Bump to 0.1.1: fix bugs, rewrite tests

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

plugins/holiday_highlighter/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "holiday_highlighter"
-version = "0.1.0"
+version = "0.1.1"
 edition = "2024"
 description = "Holiday highlighting plugin for cal using isdayoff.ru API"