feat: unify language handling across the application

- Updated the language configuration to use a single source of truth from `DEFAULT_LANGUAGE` for the bot, API, and Mini App, eliminating auto-detection from user settings.
- Refactored the `get_lang` function to always return `DEFAULT_LANGUAGE`, ensuring consistent language usage throughout the application.
- Modified the handling of language in various components, including API responses and UI elements, to reflect the new language management approach.
- Enhanced documentation and comments to clarify the changes in language handling.
- Added unit tests to verify the new language handling behavior and ensure coverage for the updated functionality.

tests/test_i18n.py

@@ -1,48 +1,46 @@
 """Unit tests for duty_teller.i18n: get_lang, t, fallback to en."""
 
-from unittest.mock import MagicMock
-
+from unittest.mock import MagicMock, patch
 
+import duty_teller.config as config
 from duty_teller.i18n import get_lang, t
 
 
-def test_get_lang_none_returns_en():
-    assert get_lang(None) == "en"
+def test_get_lang_always_returns_default_language():
+    """get_lang ignores user and always returns config.DEFAULT_LANGUAGE."""
+    assert get_lang(None) == config.DEFAULT_LANGUAGE
+    user_ru = MagicMock()
+    user_ru.language_code = "ru"
+    assert get_lang(user_ru) == config.DEFAULT_LANGUAGE
+    user_en = MagicMock()
+    user_en.language_code = "en"
+    assert get_lang(user_en) == config.DEFAULT_LANGUAGE
+    user_any = MagicMock(spec=[])
+    assert get_lang(user_any) == config.DEFAULT_LANGUAGE
 
 
-def test_get_lang_ru_returns_ru():
-    user = MagicMock()
-    user.language_code = "ru"
-    assert get_lang(user) == "ru"
+def test_get_lang_returns_ru_when_default_language_is_ru():
+    """When DEFAULT_LANGUAGE is ru, get_lang returns 'ru' regardless of user."""
+    with patch("duty_teller.i18n.core.config") as mock_cfg:
+        mock_cfg.DEFAULT_LANGUAGE = "ru"
+        from duty_teller.i18n.core import get_lang as core_get_lang
+
+        assert core_get_lang(None) == "ru"
+        user = MagicMock()
+        user.language_code = "en"
+        assert core_get_lang(user) == "ru"
 
 
-def test_get_lang_ru_ru_returns_ru():
-    user = MagicMock()
-    user.language_code = "ru-RU"
-    assert get_lang(user) == "ru"
+def test_get_lang_returns_en_when_default_language_is_en():
+    """When DEFAULT_LANGUAGE is en, get_lang returns 'en' regardless of user."""
+    with patch("duty_teller.i18n.core.config") as mock_cfg:
+        mock_cfg.DEFAULT_LANGUAGE = "en"
+        from duty_teller.i18n.core import get_lang as core_get_lang
 
-
-def test_get_lang_en_returns_en():
-    user = MagicMock()
-    user.language_code = "en"
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_uk_returns_en():
-    user = MagicMock()
-    user.language_code = "uk"
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_empty_returns_en():
-    user = MagicMock()
-    user.language_code = ""
-    assert get_lang(user) == "en"
-
-
-def test_get_lang_missing_attr_returns_en():
-    user = MagicMock(spec=[])  # no language_code
-    assert get_lang(user) == "en"
+        assert core_get_lang(None) == "en"
+        user = MagicMock()
+        user.language_code = "ru"
+        assert core_get_lang(user) == "en"
 
 
 def test_t_en_start_greeting():