✨ Add comprehensive admin panel with user management

Features added:
- Admin authentication system with is_admin field
- Complete admin dashboard with user statistics
- User management (create, edit, delete, toggle admin)
- Protected admin routes with @admin_required decorator
- Security features (prevent self-deletion, last admin protection)
- Responsive admin UI with Bootstrap integration
- Database migration script for admin field
- Admin navigation link for authorized users

Technical improvements:
- Enhanced 3-column dashboard layout (tables | editor | saved queries)
- Removed plus button and made right sidebar more compact
- Admin user (admin/admin123) automatically created with admin privileges
- Full CRUD operations for user management
- Flash messages for user feedback
- Form validation and error handling

app/__init__.py

@@ -46,7 +46,7 @@ def create_app():
         from app.models import User
         admin = User.query.filter_by(username='admin').first()
         if not admin:
-            admin = User(username='admin', email='admin@example.com')
+            admin = User(username='admin', email='admin@example.com', is_admin=True)
             admin.set_password('admin123')
             db.session.add(admin)
             db.session.commit()