feat: Simplify header transition animations in MainScreen for smoother navigation

app/src/main/java/com/rosetta/messenger/MainActivity.kt

@@ -231,34 +231,18 @@ fun MainScreen(
     AnimatedContent(
         targetState = Triple(selectedUser, showSearchScreen, Unit),
         transitionSpec = {
-            // Плавный crossfade для избежания "прыжков" header'а
-            val enterAnim = fadeIn(
-                animationSpec = tween(
-                    durationMillis = 250,
-                    easing = FastOutSlowInEasing
-                )
-            ) + slideInHorizontally(
-                initialOffsetX = { it / 4 },
-                animationSpec = tween(
-                    durationMillis = 300,
-                    easing = FastOutSlowInEasing
-                )
-            )
-            
-            val exitAnim = fadeOut(
+            // Только плавный fade без смещения - чтобы header не прыгал
+            fadeIn(
                 animationSpec = tween(
                     durationMillis = 200,
                     easing = FastOutSlowInEasing
                 )
-            ) + slideOutHorizontally(
-                targetOffsetX = { -it / 6 },
+            ) togetherWith fadeOut(
                 animationSpec = tween(
-                    durationMillis = 300,
+                    durationMillis = 150,
                     easing = FastOutSlowInEasing
                 )
-            )
-            
-            enterAnim togetherWith exitAnim using SizeTransform(clip = false)
+            ) using SizeTransform(clip = false)
         },
         label = "screenNavigation"
     ) { (user, isSearchOpen, _) ->