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>

migrations/versions/20251205_add_levels_by_language.py

@@ -0,0 +1,27 @@
+"""add levels_by_language JSON field to users
+
+Revision ID: 20251205_levels_by_lang
+Revises: 20251204_add_vocab_lang
+Create Date: 2025-12-05
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import JSON
+
+
+# revision identifiers, used by Alembic.
+revision = '20251205_levels_by_lang'
+down_revision = '20251204_add_vocab_lang'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Добавляем JSON поле для хранения уровней по языкам
+    # Формат: {"en": "B1", "ja": "N4", ...}
+    op.add_column('users', sa.Column('levels_by_language', JSON, nullable=True))
+
+
+def downgrade():
+    op.drop_column('users', 'levels_by_language')