feat: JLPT levels for Japanese, custom practice scenarios, UI improvements

- Add separate level systems: CEFR (A1-C2) for European languages, JLPT (N5-N1) for Japanese
- Store levels per language in new `levels_by_language` JSON field
- Add custom scenario option in AI practice mode
- Show action buttons after practice ends (new dialogue, tasks, words)
- Fix level display across all handlers to use correct level system
- Add Alembic migration for levels_by_language field
- Update all locale files (ru, en, ja) with new keys

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

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

locales/en.json

@@ -81,7 +81,15 @@
     "end_keep": "Great job! Keep practicing.",
     "end_hint": "Use /practice to start a new dialogue.",
     "translation_unavailable": "Translation unavailable",
-    "translation_already": "Translation already shown"
+    "translation_already": "Translation already shown",
+    "custom_scenario_btn": "✏️ Custom scenario",
+    "custom_scenario_prompt": "✏️ <b>Describe your scenario</b>\n\nWrite a topic or situation for the conversation.\n\nExamples:\n• Job interview for a programmer position\n• Ordering pizza by phone\n• Discussing a movie with a friend\n• Planning a trip to Japan",
+    "custom_scenario_too_short": "⚠️ Description too short. Write at least a few words about the scenario.",
+    "new_practice_btn": "🔄 New dialogue",
+    "to_tasks_btn": "🧠 Tasks",
+    "to_words_btn": "🎯 Words",
+    "go_tasks_hint": "Use /task to practice words",
+    "go_words_hint": "Use /words [topic] for word sets"
   },
   "tasks": {
     "no_words": "📚 You don't have words to practice yet!\n\nAdd some words with /add and come back.",
@@ -159,6 +167,87 @@
     "cancelled": "❌ Test cancelled",
     "q_header": "❓ <b>Question {i} of {n}</b>"
   },
+  "settings": {
+    "title": "⚙️ <b>Settings</b>\n\n",
+    "level_prefix": "📊 Level: ",
+    "learning_prefix": "🎯 Learning language: ",
+    "interface_prefix": "🌐 Interface language: ",
+    "choose": "Choose what to change:",
+    "close": "❌ Close",
+    "back": "⬅️ Back",
+    "back_to_settings": "⬅️ Back to settings",
+    "level_title": "📊 <b>Choose your level:</b>\n\n",
+    "level_groups": "<b>A1-A2</b> - Beginner\n<b>B1-B2</b> - Intermediate\n<b>C1-C2</b> - Advanced\n\n",
+    "level_hint": "This affects difficulty of suggested words and tasks.",
+    "level": {
+      "a1": "A1 - Beginner",
+      "a2": "A2 - Elementary",
+      "b1": "B1 - Intermediate",
+      "b2": "B2 - Upper-intermediate",
+      "c1": "C1 - Advanced",
+      "c2": "C2 - Proficient"
+    },
+    "jlpt": {
+      "n5": "N5 - Basic",
+      "n4": "N4 - Elementary",
+      "n3": "N3 - Intermediate",
+      "n2": "N2 - Advanced",
+      "n1": "N1 - Fluent"
+    },
+    "jlpt_groups": "<b>N5-N4</b> - Beginner\n<b>N3</b> - Intermediate\n<b>N2-N1</b> - Advanced\n\n",
+    "level_changed": "✅ Level changed to <b>{level}</b>\n\n",
+    "level_changed_hint": "You will now receive words and tasks matching your level!",
+    "lang_title": "🌐 <b>Select interface language:</b>\n\n",
+    "lang_desc": "This will change the language of bot messages.",
+    "lang_changed": "✅ Interface language: <b>English</b>",
+    "learning_title": "🎯 <b>Select learning language:</b>\n\n",
+    "learning_changed": "✅ Learning language: <b>{code}</b>",
+    "menu_updated": "Main menu updated ⤵️",
+    "lang_name": {
+      "ru": "🇷🇺 Русский",
+      "en": "🇬🇧 English",
+      "ja": "🇯🇵 日本語"
+    },
+    "learning_lang": {
+      "en": "🇬🇧 English",
+      "es": "🇪🇸 Spanish",
+      "de": "🇩🇪 German",
+      "fr": "🇫🇷 French",
+      "ja": "🇯🇵 Japanese"
+    }
+  },
+  "import_extra": {
+    "cancelled": "❌ Import cancelled."
+  },
+  "level_test_extra": {
+    "generating": "🔄 Generating questions...",
+    "generate_failed": "❌ Failed to generate test. Try later or use /settings to set level manually.",
+    "translation_unavailable": "Translation unavailable",
+    "translation_marker": "Question translation:",
+    "translation_already": "Translation already shown",
+    "correct": "✅ Correct!",
+    "incorrect": "❌ Incorrect",
+    "correct_answer": "Correct answer: <b>{answer}</b>",
+    "result_title": "🎉 <b>Test completed!</b>\n\n",
+    "results_header": "📊 Results:\n",
+    "correct_count": "Correct answers: <b>{correct}</b> of {total}\n",
+    "accuracy": "Accuracy: <b>{accuracy}%</b>\n\n",
+    "your_level": "🎯 Your level: <b>{level}</b>\n",
+    "level_set_hint": "Tasks and materials will now be tailored to your level!\nYou can change the level anytime via /settings",
+    "level_desc": {
+      "A1": "Beginner - understand basic phrases and can introduce yourself",
+      "A2": "Elementary - can communicate on simple topics",
+      "B1": "Intermediate - can maintain conversations on familiar topics",
+      "B2": "Upper-intermediate - fluent in most situations",
+      "C1": "Advanced - use language flexibly and effectively",
+      "C2": "Proficient - mastery at native level",
+      "N5": "Basic - understand hiragana, katakana and basic kanji",
+      "N4": "Elementary - understand everyday conversations",
+      "N3": "Intermediate - understand common texts and conversations",
+      "N2": "Advanced - understand most content",
+      "N1": "Fluent - full proficiency in Japanese"
+    }
+  },
   "words": {
     "generating": "🔄 Generating words for topic '{theme}'...",
     "generate_failed": "❌ Failed to generate words. Please try again later.",