refactor: improve language normalization and date handling utilities

- Introduced a new `normalize_lang` function to standardize language codes across the application, ensuring consistent handling of user language preferences.
- Refactored date handling utilities by adding `parse_utc_iso` and `parse_utc_iso_naive` functions for better parsing of ISO 8601 date strings, enhancing timezone awareness.
- Updated various modules to utilize the new language normalization and date parsing functions, improving code clarity and maintainability.
- Enhanced error handling in date validation to raise specific `DateRangeValidationError` exceptions, providing clearer feedback on validation issues.
- Improved test coverage for date range validation and language normalization functionalities, ensuring robustness and reliability.

duty_teller/utils/handover.py

@@ -20,13 +20,8 @@ def parse_handover_time(text: str) -> tuple[int, int] | None:
     tz_str = (m.group(4) or "").strip()
     if not tz_str or tz_str.upper() == "UTC":
         return (hour % 24, minute)
-    try:
-        from zoneinfo import ZoneInfo
-    except ImportError:
-        try:
-            from backports.zoneinfo import ZoneInfo  # type: ignore
-        except ImportError:
-            return None
+    from zoneinfo import ZoneInfo
+
     try:
         tz = ZoneInfo(tz_str)
     except Exception: