Replace Bootstrap with BeerCSS custom elements for Material Design 3 interface

[Copilot]

This PR completely migrates the application from Bootstrap 5.3.3 to BeerCSS 3.11.33, implementing a modern Material Design 3 interface as requested in the issue.

Key Changes

Infrastructure Updates

• Removed Bootstrap CDN dependency from index.html
• Added BeerCSS import in main.tsx using the npm package already in dependencies
• Updated all components to use BeerCSS classes and Material Design patterns

Component Migrations

Navigation (NavBar.tsx)

• Replaced Bootstrap navbar with BeerCSS top navigation
• Implemented responsive drawer menu with hamburger toggle
• Added Material Design icons (menu, link) for enhanced UX
• Maintained all existing functionality including search form

Layout System

• Converted Bootstrap grid (.container/.row/.col-*) to BeerCSS grid system (.grid with .s12/.m6/.l4 breakpoints)
• Updated App.tsx to use semantic <main> with BeerCSS grid container
• Applied responsive design principles across all pages

Page Components

• HomePage: Replaced jumbotron with centered hero section, converted buttons to Material Design style with .button.large.fill
• Users page: Transformed Bootstrap panels to BeerCSS cards with responsive grid layout
• User page: Updated two-column layout using BeerCSS cards and grid system
• Gists page: Simplified table styling using clean BeerCSS table design
• Loading: Replaced custom spinner with BeerCSS circular progress indicator

Benefits

• Performance: ~10x smaller bundle size compared to Bootstrap
• Modern Design: Material Design 3 compliance with built-in theming
• Accessibility: Semantic HTML elements and proper ARIA support
• Responsive: Mobile-first approach with intuitive drawer navigation
• Maintainability: Cleaner CSS classes and reduced markup complexity

The application now provides a cohesive Material Design experience while preserving all existing functionality and improving the overall user interface.

Fixes #5.

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