fix: simplify logic for determining forward/backward transition in main activity

Author: itsaky

app/src/main/java/com/itsaky/androidide/activities/MainActivity.kt

@@ -184,16 +184,7 @@ class MainActivity : LimitlessIDEActivity() {
         MaterialSharedAxis.Y
       }
 
-      val isForward = when {
-        previous == SCREEN_MAIN && screen == SCREEN_TEMPLATE_LIST -> true
-        previous == SCREEN_TEMPLATE_LIST && screen == SCREEN_TEMPLATE_DETAILS -> true
-        previous == SCREEN_TEMPLATE_DETAILS && screen == SCREEN_TEMPLATE_LIST -> false
-        previous == SCREEN_TEMPLATE_DETAILS && screen == SCREEN_MAIN -> false
-        previous == SCREEN_TEMPLATE_LIST && screen == SCREEN_MAIN -> false
-        else -> throw IllegalStateException(
-          "Invalid screen states. Previous: $previous Current: $screen"
-        )
-      }
+      val isForward = (screen ?: 0) - previous == 1
 
       val transition = MaterialSharedAxis(axis, isForward)
       transition.doOnEnd {

app/src/main/java/com/itsaky/androidide/viewmodel/MainViewModel.kt

@@ -34,7 +34,13 @@ class MainViewModel : ViewModel() {
 
   companion object {
 
-    // main screens
+    // The values assigned to these variables reflect the order in which the screens are presented
+    // to the user. A screen with a lower value is displayed before a screen with a higher value.
+    // For example, SCREEN_MAIN is the first screen visible to the user, followed by SCREEN_TEMPLATE_LIST,
+    // and then SCREEN_TEMPLATE_DETAILS.
+    //
+    // These values are used as unique identifiers for the screens as well as for determining whether
+    // the screen change transition should be forward or backward.
     const val SCREEN_MAIN = 0
     const val SCREEN_TEMPLATE_LIST = 1
     const val SCREEN_TEMPLATE_DETAILS = 2