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

@@ -3,11 +3,14 @@
         xmlns:views="using:Snake.Avalonia.Views"
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
-        mc:Ignorable="d" d:DesignWidth="480" d:DesignHeight="420"
+        mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="540"
         x:Class="Snake.Avalonia.MainWindow"
-        Title="Snake"
-        Width="480"
-        Height="420"
-        CanResize="False">
+        Title="🐍 Snake"
+        MinWidth="540"
+        MinHeight="460"
+        Width="640"
+        Height="540"
+        Background="#0D1117"
+        ExtendClientAreaToDecorationsHint="False">
     <views:GameView />
 </Window>