Add sketches into model, controller and service for the backend

application/src/main/kotlin/canvas/Sketch.kt

 package canvas
-import androidx.compose.ui.geometry.Offset
-import androidx.compose.ui.graphics.Color
-import androidx.compose.ui.unit.Dp
-import androidx.compose.ui.unit.dp
 
+import kotlinx.serialization.Serializable
+
+@Serializable
 data class Sketch(
-    val start: Offset,
-    val end: Offset,
-    val color: Color = Color.Black,
-    val width: Dp = 1.dp,
+    val startX: Float,
+    val startY: Float,
+    val endX: Float,
+    val endY: Float,
+    val color: Int,
+    val width: Int,
 )
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