Admin improvements, RBAC and styling fixes

Extend admin interface:

- Make admin controllers inherit from Admin::BaseController
- New admin dashboard with system statistics overview
- Centralized admin layout with dedicated navigation
- Enhanced user management interface with role assignment capabilities

RBAC fixes:

- Introduced new `admin` role that has all rights
- Added db migration to create admin role with appropriate permissions
- Updated admin rake tasks for role management
- Updated db seeds to add all required roles properly

Styling improvements and fixes:

- Upgraded button system with standardized styling and dark mode support
- Improved UI with consistent styling using Tailwind components
- Brought back manual dark mode using Tailwind custom variant