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.

duty_teller/api/telegram_auth.py

@@ -6,7 +6,7 @@ import json
 import time
 from urllib.parse import unquote
 
-from duty_teller.i18n.lang import normalize_lang
+import duty_teller.config as config
 
 # Telegram algorithm: https://core.telegram.org/bots/webapps#validating-data-received-via-the-mini-app
 # Data-check string: sorted key=value with URL-decoded values, then HMAC-SHA256(WebAppData, token) as secret.
@@ -48,12 +48,12 @@ def validate_init_data_with_reason(
     Returns:
         Tuple (telegram_user_id, username, reason, lang). reason is one of: "ok",
         "empty", "no_hash", "hash_mismatch", "auth_date_expired", "no_user",
-        "user_invalid", "no_user_id". lang is from user.language_code normalized
-        to 'ru' or 'en'; 'en' when no user. On success: (user.id, username or None,
-        "ok", lang).
+        "user_invalid", "no_user_id". lang is always config.DEFAULT_LANGUAGE.
+        On success: (user.id, username or None, "ok", lang).
     """
+    lang = config.DEFAULT_LANGUAGE
     if not init_data or not bot_token:
-        return (None, None, "empty", "en")
+        return (None, None, "empty", lang)
     init_data = init_data.strip()
     params = {}
     for part in init_data.split("&"):
@@ -65,7 +65,7 @@ def validate_init_data_with_reason(
         params[key] = value
     hash_val = params.pop("hash", None)
     if not hash_val:
-        return (None, None, "no_hash", "en")
+        return (None, None, "no_hash", lang)
     data_pairs = sorted(params.items())
     # Data-check string: key=value with URL-decoded values (per Telegram example)
     data_string = "\n".join(f"{k}={unquote(v)}" for k, v in data_pairs)
@@ -81,27 +81,26 @@ def validate_init_data_with_reason(
         digestmod=hashlib.sha256,
     ).hexdigest()
     if not hmac.compare_digest(computed.lower(), hash_val.lower()):
-        return (None, None, "hash_mismatch", "en")
+        return (None, None, "hash_mismatch", lang)
     if max_age_seconds is not None and max_age_seconds > 0:
         auth_date_raw = params.get("auth_date")
         if not auth_date_raw:
-            return (None, None, "auth_date_expired", "en")
+            return (None, None, "auth_date_expired", lang)
         try:
             auth_date = int(float(auth_date_raw))
         except (ValueError, TypeError):
-            return (None, None, "auth_date_expired", "en")
+            return (None, None, "auth_date_expired", lang)
         if time.time() - auth_date > max_age_seconds:
-            return (None, None, "auth_date_expired", "en")
+            return (None, None, "auth_date_expired", lang)
     user_raw = params.get("user")
     if not user_raw:
-        return (None, None, "no_user", "en")
+        return (None, None, "no_user", lang)
     try:
         user = json.loads(unquote(user_raw))
     except (json.JSONDecodeError, TypeError):
-        return (None, None, "user_invalid", "en")
+        return (None, None, "user_invalid", lang)
     if not isinstance(user, dict):
-        return (None, None, "user_invalid", "en")
-    lang = normalize_lang(user.get("language_code"))
+        return (None, None, "user_invalid", lang)
     raw_id = user.get("id")
     if raw_id is None:
         return (None, None, "no_user_id", lang)