Fix image zoom focal point

app/androidApp/src/test/kotlin/com/example/aiapp/SessionImageTest.kt

@@ -0,0 +1,37 @@
+package com.example.aiapp
+
+import androidx.compose.ui.geometry.Offset
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class SessionImageTest {
+    @Test
+    fun `zoom keeps the region panned to in the center`() {
+        assertEquals(
+            Offset(240f, -160f),
+            zoomOffset(
+                offset = Offset(120f, -80f),
+                centroid = Offset(500f, 1000f),
+                pan = Offset.Zero,
+                oldScale = 2f,
+                newScale = 4f,
+                viewportCenter = Offset(500f, 1000f),
+            ),
+        )
+    }
+
+    @Test
+    fun `zoom keeps an off-center pinch beneath moving fingers`() {
+        assertEquals(
+            Offset(460f, 420f),
+            zoomOffset(
+                offset = Offset(100f, -100f),
+                centroid = Offset(250f, 400f),
+                pan = Offset(10f, 20f),
+                oldScale = 2f,
+                newScale = 4f,
+                viewportCenter = Offset(500f, 1000f),
+            ),
+        )
+    }
+}