Refactor app navigation to use Navigation3

Integrate `navigation3` into `AppScreen` to manage tab transitions and
backstack state. Replaced manual tab state management with `NavDisplay`
and `NavBackStack`, and added horizontal slide animations for switching
between Home, Libraries, and Downloads screens.

app/src/main/java/hu/bbara/purefin/ui/screen/AppScreen.kt

@@ -1,34 +1,52 @@
 package hu.bbara.purefin.ui.screen
 
+import androidx.compose.animation.AnimatedContentTransitionScope
+import androidx.compose.animation.ContentTransform
+import androidx.compose.animation.core.tween
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.remember
-import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.IntOffset
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.lifecycle.compose.LifecycleResumeEffect
+import androidx.navigation3.runtime.NavBackStack
+import androidx.navigation3.runtime.NavKey
+import androidx.navigation3.runtime.entryProvider
+import androidx.navigation3.runtime.rememberNavBackStack
+import androidx.navigation3.runtime.rememberSaveableStateHolderNavEntryDecorator
+import androidx.navigation3.scene.Scene
+import androidx.navigation3.ui.NavDisplay
 import hu.bbara.purefin.feature.browse.home.AppViewModel
+import hu.bbara.purefin.navigation.LocalNavigationManager
 import hu.bbara.purefin.ui.screen.download.DownloadsScreen
 import hu.bbara.purefin.ui.screen.home.HomeScreen
 import hu.bbara.purefin.ui.screen.home.components.HomeNavItem
 import hu.bbara.purefin.ui.screen.libraries.LibrariesScreen
+import kotlinx.serialization.Serializable
+import androidx.compose.animation.slideInHorizontally
+import androidx.compose.animation.slideOutHorizontally
+import androidx.compose.animation.togetherWith
 
 @Composable
 fun AppScreen(
     viewModel: AppViewModel = hiltViewModel(),
     modifier: Modifier = Modifier
 ) {
-    var selectedTab by remember { mutableIntStateOf(0) }
-
     val libraries by viewModel.libraries.collectAsState()
     val libraryContent by viewModel.latestLibraryContent.collectAsState()
     val suggestions by viewModel.suggestions.collectAsState()
     val continueWatching by viewModel.continueWatching.collectAsState()
     val nextUp by viewModel.nextUp.collectAsState()
     val isRefreshing by viewModel.isRefreshing.collectAsState()
+    val navigationManager = LocalNavigationManager.current
+
+    @Suppress("UNCHECKED_CAST")
+    val backStack = rememberNavBackStack(AppTabRoute.Home) as NavBackStack<AppTabRoute>
+    val currentRoute = backStack.lastOrNull() ?: AppTabRoute.Home
+    val selectedTab = currentRoute.toTabIndex()
 
     val libraryNavItems = libraries.map {
         HomeNavItem(
@@ -43,8 +61,18 @@ fun AppScreen(
         onPauseOrDispose { }
     }
 
-    when (selectedTab) {
+    val onTabSelected = remember(backStack) {
-        0 -> HomeScreen(
+        { selectedIndex: Int ->
+            val route = selectedIndex.toAppTabRoute()
+            if (backStack.lastOrNull() != route) {
+                backStack.add(route)
+            }
+        }
+    }
+
+    val tabEntryProvider = entryProvider {
+        entry<AppTabRoute.Home>(metadata = appTabMetadata(AppTabRoute.Home)) {
+            HomeScreen(
                 libraries = libraries,
                 libraryContent = libraryContent,
                 suggestions = suggestions,
@@ -63,23 +91,101 @@ fun AppScreen(
                 onSettingsClick = {},
                 onLogoutClick = viewModel::logout,
                 selectedTab = selectedTab,
-            onTabSelected = { selectedTab = it },
+                onTabSelected = onTabSelected,
-            modifier = modifier.fillMaxSize()
+                modifier = Modifier.fillMaxSize()
             )
-        1 -> LibrariesScreen(
+        }
+        entry<AppTabRoute.Libraries>(metadata = appTabMetadata(AppTabRoute.Libraries)) {
+            LibrariesScreen(
                 items = libraryNavItems,
                 onLibrarySelected = { item -> viewModel.onLibrarySelected(item.id, item.label) },
                 onProfileClick = {},
                 onSettingsClick = {},
                 onLogoutClick = viewModel::logout,
                 selectedTab = selectedTab,
-            onTabSelected = { selectedTab = it },
+                onTabSelected = onTabSelected,
-            modifier = modifier.fillMaxSize()
+                modifier = Modifier.fillMaxSize()
-        )
-        2 -> DownloadsScreen(
-            selectedTab = selectedTab,
-            onTabSelected = { selectedTab = it },
-            modifier = modifier.fillMaxSize()
             )
         }
+        entry<AppTabRoute.Downloads>(metadata = appTabMetadata(AppTabRoute.Downloads)) {
+            DownloadsScreen(
+                selectedTab = selectedTab,
+                onTabSelected = onTabSelected,
+                modifier = Modifier.fillMaxSize()
+            )
+        }
+    }
+
+    NavDisplay(
+        backStack = backStack,
+        onBack = {
+            if (backStack.size > 1) {
+                backStack.removeLastOrNull()
+            } else {
+                navigationManager.pop()
+            }
+        },
+        entryDecorators = listOf(
+            rememberSaveableStateHolderNavEntryDecorator()
+        ),
+        transitionSpec = { appTabTransition() },
+        popTransitionSpec = { appTabTransition() },
+        predictivePopTransitionSpec = { _ -> appTabTransition() },
+        entryProvider = tabEntryProvider,
+        modifier = modifier.fillMaxSize()
+    )
 }
+
+@Serializable
+private sealed interface AppTabRoute : NavKey {
+    @Serializable
+    data object Home : AppTabRoute
+
+    @Serializable
+    data object Libraries : AppTabRoute
+
+    @Serializable
+    data object Downloads : AppTabRoute
+}
+
+private fun AppTabRoute.toTabIndex(): Int = when (this) {
+    AppTabRoute.Home -> 0
+    AppTabRoute.Libraries -> 1
+    AppTabRoute.Downloads -> 2
+}
+
+private fun Int.toAppTabRoute(): AppTabRoute = when (this) {
+    0 -> AppTabRoute.Home
+    1 -> AppTabRoute.Libraries
+    2 -> AppTabRoute.Downloads
+    else -> AppTabRoute.Home
+}
+
+private fun AnimatedContentTransitionScope<Scene<AppTabRoute>>.appTabTransition(): ContentTransform {
+    val initialIndex = initialState.metadata.appTabIndex()
+    val targetIndex = targetState.metadata.appTabIndex()
+    val animationSpec = tween<IntOffset>(durationMillis = 140)
+
+    return when {
+        targetIndex > initialIndex -> {
+            slideInHorizontally(animationSpec = animationSpec) { fullWidth -> fullWidth } togetherWith
+                slideOutHorizontally(animationSpec = animationSpec) { fullWidth -> -fullWidth }
+        }
+        targetIndex < initialIndex -> {
+            slideInHorizontally(animationSpec = animationSpec) { fullWidth -> -fullWidth } togetherWith
+                slideOutHorizontally(animationSpec = animationSpec) { fullWidth -> fullWidth }
+        }
+        else -> {
+            slideInHorizontally(animationSpec = animationSpec) { 0 } togetherWith
+                slideOutHorizontally(animationSpec = animationSpec) { 0 }
+        }
+    }
+}
+
+private fun appTabMetadata(route: AppTabRoute): Map<String, Any> =
+    mapOf(APP_TAB_INDEX_METADATA to route.toTabIndex())
+
+private fun Map<String, Any>.appTabIndex(): Int =
+    this[APP_TAB_INDEX_METADATA] as? Int ?: 0
+
+private const val APP_TAB_INDEX_METADATA = "app_tab_index"