refactor: vertically align Add Task screen

resolves issue on iOS where a new task couldn’t be added because the button is hidden behind the keyboard

Author: phatblat

kotlin-multiplatform/composeApp/src/commonMain/kotlin/com/ditto/quickstart/ui/TaskAddEditScreen.kt

@@ -47,7 +47,7 @@ fun TaskAddEditScreen(
 
     Column(
         modifier = modifier.padding(horizontal = 16.dp),
-        verticalArrangement = Arrangement.Center
+        verticalArrangement = Arrangement.Top,
     ) {
         Text(
             text = title,