Modernize GUI with enhanced styling and user experience improvements

- Added comprehensive modern color scheme with professional palette
- Created branded title bar with app logo and status indicators
- Enhanced repository panel with card-based layout design
- Implemented modern button styles (Primary, Success, Danger, Action)
- Added improved typography and spacing throughout interface
- Created bottom status bar with progress indicators
- Enhanced treeview and form controls with modern styling
- Fixed layout issues and improved visual hierarchy
- Added better visual feedback and interaction patterns

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

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

Author: dev-alt