[FEAT] - Custom window chrome for modern UI aesthetic

[dylanlangston]

Is your feature request related to a problem? Please describe.

The application currently uses default OS window chrome (title bar and borders), which doesn't provide a cohesive, modern aesthetic that matches the app's comic-inspired personality. Custom window chrome would give AvaloniaXKCD a more polished, distinctive appearance.

Describe the solution you'd like

Design and implement custom window chrome for AvaloniaXKCD following modern Avalonia best practices:

• Follow the Avalonia UI tutorial on creating a modern-looking window
• Research Avalonia's WindowChrome and ExtendClientArea APIs
• Design and implement a custom title bar and window frame in XAML that aligns with the application's aesthetic
• Create a reusable custom title bar control (buttons, drag region, app name/logo)
• Ensure all standard window controls (minimize, maximize, close) are correctly wired to host OS logic
• Add animations or visual polish where appropriate (hover, active states)
• Ensure window behavior (drag, resize, minimize/maximize/close) works correctly on all desktop platforms
• Test under dark/light modes and different DPI settings
• Verify custom chrome behaves correctly across all desktop targets (AvaloniaXKCD.Desktop, potential macOS/Linux ports)
• Document the structure and styling for future customization

References:

• Avalonia Custom Window Chrome Guide

Describe alternatives you've considered

• Keeping default OS chrome, but this provides less visual distinctiveness.

Additional context

Acceptance Criteria:

• Default OS chrome is replaced by custom Avalonia layout
• All window interactions work as expected (drag, resize, min/max/close)
• Visual design is consistent, modern, and cross-platform
• Chrome works correctly in both dark and light modes
• High DPI displays are properly supported
• Code and styling are well-documented for future customization