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