Refactor settings system and improve context initialization

Backend:
- Add type validation and coercion for settings API
- Implement SettingStorage and SettingType in registry
- Improve CRUD operations for settings

Frontend:
- Refactor Theme, Language, Sidebar, ViewMode contexts
- Simplify admin components (GeneralTab, SettingsTab, UsersTab)
- Add new settings endpoints to API client
- Improve App initialization flow

Infrastructure:
- Update Dockerfile and docker-compose.yml
- Add .dockerignore
- Update Makefile and README

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/env.py

@@ -15,6 +15,7 @@ from app.config import settings
 
 # Import all models so Alembic can detect them for auto-generating migrations
 from app.models.user import User  # noqa
+from app.models.settings import Settings  # noqa
 
 # this is the Alembic Config object, which provides
 # access to the values within the .ini file in use.