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_telegram_auth.py

@@ -1,5 +1,8 @@
 """Tests for duty_teller.api.telegram_auth.validate_init_data and validate_init_data_with_reason."""
 
+from unittest.mock import patch
+
+import duty_teller.config as config
 from duty_teller.api.telegram_auth import (
     validate_init_data,
     validate_init_data_with_reason,
@@ -52,7 +55,7 @@ def test_user_without_username_returns_none_from_validate_init_data():
 
 
 def test_user_without_username_but_with_id_succeeds_with_reason():
-    """With validate_init_data_with_reason, valid user.id is enough; username may be None."""
+    """With validate_init_data_with_reason, valid user.id is enough; lang is DEFAULT_LANGUAGE."""
     bot_token = "123:ABC"
     user = {"id": 456, "first_name": "Test", "language_code": "ru"}
     init_data = make_init_data(user, bot_token)
@@ -62,11 +65,11 @@ def test_user_without_username_but_with_id_succeeds_with_reason():
     assert telegram_user_id == 456
     assert username is None
     assert reason == "ok"
-    assert lang == "ru"
+    assert lang == config.DEFAULT_LANGUAGE
 
 
 def test_user_without_id_returns_no_user_id():
-    """When user object exists but has no 'id', return no_user_id."""
+    """When user object exists but has no 'id', return no_user_id; lang is DEFAULT_LANGUAGE."""
     bot_token = "123:ABC"
     user = {"first_name": "Test"}  # no id
     init_data = make_init_data(user, bot_token)
@@ -76,7 +79,17 @@ def test_user_without_id_returns_no_user_id():
     assert telegram_user_id is None
     assert username is None
     assert reason == "no_user_id"
-    assert lang == "en"
+    assert lang == config.DEFAULT_LANGUAGE
+
+
+def test_validate_init_data_with_reason_returns_default_language_ignoring_user_lang():
+    """Returned lang is always config.DEFAULT_LANGUAGE, not user.language_code."""
+    with patch("duty_teller.api.telegram_auth.config.DEFAULT_LANGUAGE", "ru"):
+        user = {"id": 1, "first_name": "U", "language_code": "en"}
+        init_data = make_init_data(user, "123:ABC")
+        _, _, reason, lang = validate_init_data_with_reason(init_data, "123:ABC")
+        assert reason == "ok"
+        assert lang == "ru"
 
 
 def test_empty_init_data_returns_none():