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/App.axaml

@@ -1,10 +1,38 @@
 <Application xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:Class="Snake.Avalonia.App"
-             RequestedThemeVariant="Default">
-             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->
+             RequestedThemeVariant="Dark">
 
     <Application.Styles>
         <FluentTheme />
+
+        <!-- Custom styles for game buttons -->
+        <Style Selector="Button.GameButton">
+            <Setter Property="Background" Value="#0D9488" />
+            <Setter Property="Foreground" Value="#E6EDF3" />
+            <Setter Property="FontSize" Value="14" />
+            <Setter Property="FontWeight" Value="Bold" />
+            <Setter Property="CornerRadius" Value="8" />
+            <Setter Property="Padding" Value="16,8" />
+            <Setter Property="BorderThickness" Value="0" />
+        </Style>
+
+        <Style Selector="Button.GameButton:pointerover">
+            <Setter Property="Background" Value="#14B8A6" />
+        </Style>
+
+        <Style Selector="Button.GameButton:pressed">
+            <Setter Property="Background" Value="#0F766E" />
+        </Style>
+
+        <Style Selector="Button.GameButtonSecondary">
+            <Setter Property="Background" Value="#1C2128" />
+            <Setter Property="BorderBrush" Value="#30363D" />
+            <Setter Property="BorderThickness" Value="1" />
+        </Style>
+
+        <Style Selector="Button.GameButtonSecondary:pointerover">
+            <Setter Property="Background" Value="#30363D" />
+        </Style>
     </Application.Styles>
-</Application>
+</Application>