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>

locales/en.json

@@ -202,6 +202,7 @@
     "level_prefix": "📊 Level: ",
     "learning_prefix": "🎯 Learning language: ",
     "interface_prefix": "🌐 Interface language: ",
+    "translation_prefix": "💬 Translation language: ",
     "choose": "Choose what to change:",
     "close": "❌ Close",
     "back": "⬅️ Back",
@@ -232,6 +233,9 @@
     "lang_changed": "✅ Interface language: <b>English</b>",
     "learning_title": "🎯 <b>Select learning language:</b>\n\n",
     "learning_changed": "✅ Learning language: <b>{code}</b>",
+    "translation_title": "💬 <b>Select translation language:</b>\n\n",
+    "translation_desc": "Words will be translated to this language.\nThis can differ from interface language.",
+    "translation_changed": "✅ Translation language: <b>{lang_name}</b>",
     "menu_updated": "Main menu updated ⤵️",
     "lang_name": {
       "ru": "🇷🇺 Русский",
@@ -290,6 +294,13 @@
       "N1": "Fluent - full proficiency in Japanese"
     }
   },
+  "onboarding": {
+    "step2_title": "🎯 Which language do you want to learn?",
+    "step3_title": "💬 Which language to translate words into?",
+    "complete": "✅ Settings saved!",
+    "lang_en": "🇬🇧 English",
+    "lang_ja": "🇯🇵 Japanese"
+  },
   "words": {
     "generating": "🔄 Generating words for topic '{theme}'...",
     "generate_failed": "❌ Failed to generate words. Please try again later.",