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/Views/HighScoreManager.cs

@@ -0,0 +1,58 @@
+using System.Text.Json;
+
+namespace Snake.Avalonia.Views;
+
+/// <summary>
+/// Persists high score to a JSON file in the user's local app data.
+/// </summary>
+public static class HighScoreManager
+{
+    private static readonly string FilePath;
+
+    static HighScoreManager()
+    {
+        var appData = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
+        var dir = Path.Combine(appData, "SnakeAvalonia");
+        Directory.CreateDirectory(dir);
+        FilePath = Path.Combine(dir, "highscore.json");
+    }
+
+    public static int Load()
+    {
+        try
+        {
+            if (File.Exists(FilePath))
+            {
+                var json = File.ReadAllText(FilePath);
+                var data = JsonSerializer.Deserialize<HighScoreData>(json);
+                return data?.Score ?? 0;
+            }
+        }
+        catch
+        {
+            // Corrupted file — reset
+        }
+
+        return 0;
+    }
+
+    public static void Save(int score)
+    {
+        try
+        {
+            var data = new HighScoreData { Score = score, Date = DateTime.UtcNow.ToString("O") };
+            var json = JsonSerializer.Serialize(data);
+            File.WriteAllText(FilePath, json);
+        }
+        catch
+        {
+            // Silently fail — high score is non-critical
+        }
+    }
+
+    private sealed class HighScoreData
+    {
+        public int Score { get; set; }
+        public string Date { get; set; } = string.Empty;
+    }
+}