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/i18n/core.py

@@ -3,6 +3,7 @@
 from typing import TYPE_CHECKING
 
 import duty_teller.config as config
+from duty_teller.i18n.lang import normalize_lang
 from duty_teller.i18n.messages import MESSAGES
 
 if TYPE_CHECKING:
@@ -12,13 +13,12 @@ if TYPE_CHECKING:
 def get_lang(user: "User | None") -> str:
     """
     Normalize Telegram user language to 'ru' or 'en'.
-    If user has language_code starting with 'ru' (e.g. ru, ru-RU) return 'ru', else 'en'.
-    When user is None or has no language_code, return config.DEFAULT_LANGUAGE.
+    Uses normalize_lang for user.language_code; when user is None or has no
+    language_code, returns config.DEFAULT_LANGUAGE.
     """
     if user is None or not getattr(user, "language_code", None):
         return config.DEFAULT_LANGUAGE
-    code = (user.language_code or "").strip().lower()
-    return "ru" if code.startswith("ru") else "en"
+    return normalize_lang(user.language_code)
 
 
 def t(lang: str, key: str, **kwargs: str) -> str: