Implement dark mode support across dashboard

Description

Add dark mode support to the web-dashboard to improve user experience and reduce eye strain.

Tasks

• Create theme context provider
• Add CSS variables for light/dark themes
• Update all components to use theme variables
• Add theme toggle in settings
• Persist theme preference in localStorage
• Add smooth transitions between themes

Related

This will use the ui-component-library for base components.

cc @stan @jean