feat: tail interpolates like head (mirrored with ease-in)

Tail now uses the same approach as head: interpolated in the body path
from its previous position, with ease-in (mirror of head's ease-out).
Ghost circle removed — no longer needed since tail is naturally smooth.
Added EaseInCubic to AnimationHelper. InterpolatePosition now uses
string easing mode instead of bool.

Snake.Avalonia/Views/AnimationHelper.cs

@@ -26,7 +26,16 @@ public static class AnimationHelper
     }
 
     /// <summary>
-    /// Interpolates between two positions with given t value.
+    /// Ease-in cubic — mirror of EaseOutCubic, for tail interpolation.
+    /// </summary>
+    public static double EaseInCubic(double t)
+    {
+        t = Math.Clamp(t, 0.0, 1.0);
+        return t * t * t;
+    }
+
+    /// <summary>
+    /// Interpolates between two positions with given t value and easing mode.
     /// Returns the visual position for rendering.
     /// </summary>
     public static (double X, double Y) InterpolatePosition(
@@ -34,9 +43,14 @@ public static class AnimationHelper
         Position to,
         double t,
         double cellSize,
-        bool useEasing = false)
+        string easing = "linear")
     {
-        var eased = useEasing ? EaseOutCubic(t) : t;
+        var eased = easing switch
+        {
+            "easeOut" => EaseOutCubic(t),
+            "easeIn" => EaseInCubic(t),
+            _ => t
+        };
         var x = from.X + (to.X - from.X) * eased;
         var y = from.Y + (to.Y - from.Y) * eased;
         return (x * cellSize + cellSize * 0.5, y * cellSize + cellSize * 0.5);