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>

database/models.py

@@ -12,7 +12,7 @@ class Base(DeclarativeBase):
 
 
 class LanguageLevel(str, enum.Enum):
-    """Уровни владения языком"""
+    """Уровни владения языком (CEFR)"""
     A1 = "A1"
     A2 = "A2"
     B1 = "B1"
@@ -21,6 +21,23 @@ class LanguageLevel(str, enum.Enum):
     C2 = "C2"
 
 
+class JLPTLevel(str, enum.Enum):
+    """Уровни JLPT для японского языка"""
+    N5 = "N5"  # Базовый
+    N4 = "N4"  # Начальный
+    N3 = "N3"  # Средний
+    N2 = "N2"  # Продвинутый
+    N1 = "N1"  # Свободный
+
+
+# Языки, использующие JLPT вместо CEFR
+JLPT_LANGUAGES = {"ja"}
+
+# Дефолтные уровни для разных систем
+DEFAULT_CEFR_LEVEL = "A1"
+DEFAULT_JLPT_LEVEL = "N5"
+
+
 class WordSource(str, enum.Enum):
     """Источник добавления слова"""
     MANUAL = "manual"  # Ручное добавление
@@ -40,6 +57,7 @@ class User(Base):
     language_interface: Mapped[str] = mapped_column(String(2), default="ru")  # ru/en
     learning_language: Mapped[str] = mapped_column(String(2), default="en")  # en
     level: Mapped[LanguageLevel] = mapped_column(SQLEnum(LanguageLevel), default=LanguageLevel.A1)
+    levels_by_language: Mapped[Optional[dict]] = mapped_column(JSON, default=None)  # {"en": "B1", "ja": "N4"}
     timezone: Mapped[str] = mapped_column(String(50), default="UTC")
     daily_task_time: Mapped[Optional[str]] = mapped_column(String(5))  # HH:MM
     reminders_enabled: Mapped[bool] = mapped_column(Boolean, default=False)