feat: add translation language setting & onboarding flow

- Add separate translation_language setting (independent from interface language)
- Implement 3-step onboarding for new users:
  1. Choose interface language
  2. Choose learning language
  3. Choose translation language
- Fix localization issues when using callback.message (user_id from state)
- Add UserService.get_user_by_id() method
- Add get_user_translation_lang() helper in i18n
- Update all handlers to use correct translation language
- Add localization keys for onboarding (ru/en/ja)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

services/user_service.py

@@ -59,6 +59,23 @@ class UserService:
         )
         return result.scalar_one_or_none()
 
+    @staticmethod
+    async def get_user_by_id(session: AsyncSession, user_id: int) -> Optional[User]:
+        """
+        Получить пользователя по внутреннему ID
+
+        Args:
+            session: Сессия базы данных
+            user_id: ID пользователя в БД
+
+        Returns:
+            Объект пользователя или None
+        """
+        result = await session.execute(
+            select(User).where(User.id == user_id)
+        )
+        return result.scalar_one_or_none()
+
     @staticmethod
     async def update_user_level(session: AsyncSession, user_id: int, level: str, language: str = None):
         """
@@ -120,3 +137,22 @@ class UserService:
         if user:
             user.learning_language = language
             await session.commit()
+
+    @staticmethod
+    async def update_user_translation_language(session: AsyncSession, user_id: int, language: str):
+        """
+        Обновить язык перевода пользователя
+
+        Args:
+            session: Сессия базы данных
+            user_id: ID пользователя
+            language: Новый язык перевода (ru/en/ja)
+        """
+        result = await session.execute(
+            select(User).where(User.id == user_id)
+        )
+        user = result.scalar_one_or_none()
+
+        if user:
+            user.translation_language = language
+            await session.commit()