feat: smooth snake animation + beautiful Avalonia UI overhaul

- Two-timer architecture: game timer + 60fps render timer for smooth interpolation
- Snake body: StreamGeometry path with teal gradient, rounded joins
- Directional head with white eyes and dark pupils
- Food: pulsating glow, highlight, green leaf animation
- Modern dark theme (#0D1117), glassmorphism HUD
- Speed indicator bar, score +N popup
- High score persistence to JSON
- All keyboard shortcuts: Arrows, WASD, Space/P pause, Enter start, R restart, Esc quit
- Window resizable, 640x540 default

New files: AnimationHelper.cs, HighScoreManager.cs, SnakeRenderer.cs

Snake.Avalonia/MainWindow.axaml.cs

@@ -1,4 +1,5 @@
 using Avalonia.Controls;
+using Avalonia.Input;
 
 namespace Snake.Avalonia;
 
@@ -7,5 +8,15 @@ public partial class MainWindow : Window
     public MainWindow()
     {
         InitializeComponent();
+
+        // Global Escape handler to close the window
+        KeyDown += (_, e) =>
+        {
+            if (e.Key == Key.Escape)
+            {
+                Close();
+                e.Handled = true;
+            }
+        };
     }
-}
+}