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.

tests/test_utils.py

@@ -5,6 +5,7 @@ from datetime import date
 import pytest
 
 from duty_teller.utils.dates import (
+    DateRangeValidationError,
     day_start_iso,
     day_end_iso,
     duty_to_iso,
@@ -47,15 +48,18 @@ def test_validate_date_range_ok():
 
 
 def test_validate_date_range_bad_format():
-    with pytest.raises(ValueError, match="формате YYYY-MM-DD"):
+    with pytest.raises(DateRangeValidationError) as exc_info:
         validate_date_range("01-01-2025", "2025-01-31")
-    with pytest.raises(ValueError, match="формате YYYY-MM-DD"):
+    assert exc_info.value.kind == "bad_format"
+    with pytest.raises(DateRangeValidationError) as exc_info:
         validate_date_range("2025-01-01", "invalid")
+    assert exc_info.value.kind == "bad_format"
 
 
 def test_validate_date_range_from_after_to():
-    with pytest.raises(ValueError, match="from не должна быть позже"):
+    with pytest.raises(DateRangeValidationError) as exc_info:
         validate_date_range("2025-02-01", "2025-01-01")
+    assert exc_info.value.kind == "from_after_to"
 
 
 # --- user ---