refactor(folio): replace custom Navigator with NavHost backstack

Wraps androidx.navigation.NavHostController behind the existing
push/replace/back surface so call sites in ViewModels stay unchanged.
App.kt now wires a typed NavHost with @Serializable Route entries
and observes the controller's currentBackStackEntry to drive the
session-based Login/Home redirect.

examples/folio/app/shared/src/commonMain/kotlin/app/folio/App.kt

@@ -2,7 +2,6 @@ package app.folio
 
 import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Box
-import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.WindowInsets
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.safeDrawing
@@ -17,6 +16,12 @@ import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.navigation.NavDestination.Companion.hasRoute
+import androidx.navigation.compose.NavHost
+import androidx.navigation.compose.composable
+import androidx.navigation.compose.currentBackStackEntryAsState
+import androidx.navigation.compose.rememberNavController
+import androidx.navigation.toRoute
 import app.folio.di.AppGraph
 import app.folio.di.LocalAppGraph
 import app.folio.feature.account.AddAccountRoute
@@ -61,24 +66,35 @@ fun App(graphBuilder: suspend () -> AppGraph) {
 @Composable
 private fun AppContent() {
     val graph = LocalAppGraph.current
-    val session by graph.repository.session.collectAsState()
-    val route by graph.navigator.current.collectAsState()
+    val navController = rememberNavController()
 
-    LaunchedEffect(session, route) {
-        if (session == null && route !is Route.Login) {
+    LaunchedEffect(navController) { graph.navigator.attach(navController) }
+
+    val session by graph.repository.session.collectAsState()
+    val currentEntry by navController.currentBackStackEntryAsState()
+    val onLogin = currentEntry?.destination?.hasRoute(Route.Login::class) == true
+
+    LaunchedEffect(session, onLogin) {
+        if (session == null && !onLogin) {
             graph.navigator.replace(Route.Login)
-        } else if (session != null && route is Route.Login) {
+        } else if (session != null && onLogin) {
             graph.navigator.replace(Route.Home)
         }
     }
 
-    Column(Modifier.fillMaxSize()) {
-        when (val r = route) {
-            Route.Login -> LoginRoute()
-            Route.Home -> HomeRoute()
-            Route.AddAccount -> AddAccountRoute()
-            is Route.Ledger -> LedgerRoute(accountId = r.accountId)
-            is Route.AddTransaction -> AddTransactionRoute(accountId = r.accountId)
+    NavHost(
+        navController = navController,
+        startDestination = Route.Home,
+        modifier = Modifier.fillMaxSize(),
+    ) {
+        composable<Route.Login> { LoginRoute() }
+        composable<Route.Home> { HomeRoute() }
+        composable<Route.AddAccount> { AddAccountRoute() }
+        composable<Route.Ledger> { entry ->
+            LedgerRoute(accountId = entry.toRoute<Route.Ledger>().accountId)
+        }
+        composable<Route.AddTransaction> { entry ->
+            AddTransactionRoute(accountId = entry.toRoute<Route.AddTransaction>().accountId)
         }
     }
 }

examples/folio/app/shared/src/commonMain/kotlin/app/folio/di/AppGraph.kt

@@ -10,7 +10,6 @@ import app.folio.feature.home.HomeViewModel
 import app.folio.feature.ledger.AddTransactionViewModel
 import app.folio.feature.ledger.LedgerViewModel
 import app.folio.navigation.Navigator
-import app.folio.navigation.Route
 import dev.zacsweers.metro.AppScope
 import dev.zacsweers.metro.Binds
 import dev.zacsweers.metro.DependencyGraph
@@ -33,7 +32,7 @@ interface AppGraph {
 
     @SingleIn(AppScope::class)
     @Provides
-    fun provideNavigator(): Navigator = Navigator(initial = Route.Home)
+    fun provideNavigator(): Navigator = Navigator()
 
     @DependencyGraph.Factory
     fun interface Factory {

examples/folio/app/shared/src/commonMain/kotlin/app/folio/navigation/Navigator.kt

@@ -1,30 +1,31 @@
 package app.folio.navigation
 
-import kotlinx.coroutines.flow.MutableStateFlow
-import kotlinx.coroutines.flow.StateFlow
-import kotlinx.coroutines.flow.asStateFlow
+import androidx.navigation.NavHostController
 
-class Navigator(initial: Route = Route.Home) {
-    private val stack = ArrayDeque<Route>().apply { addLast(initial) }
-    private val _current = MutableStateFlow<Route>(initial)
-    val current: StateFlow<Route> = _current.asStateFlow()
+class Navigator {
+    private var controller: NavHostController? = null
+
+    fun attach(controller: NavHostController) {
+        this.controller = controller
+    }
 
     fun push(route: Route) {
-        stack.addLast(route)
-        _current.value = route
+        controller?.navigate(route) {
+            launchSingleTop = true
+        }
     }
 
     fun replace(route: Route) {
-        stack.clear()
-        stack.addLast(route)
-        _current.value = route
+        val nav = controller ?: return
+        nav.navigate(route) {
+            popUpTo(nav.graph.id) { inclusive = true }
+            launchSingleTop = true
+        }
     }
 
     fun back(fallback: Route) {
-        if (stack.size > 1) {
-            stack.removeLast()
-            _current.value = stack.last()
-        } else {
+        val nav = controller ?: return
+        if (!nav.popBackStack()) {
             replace(fallback)
         }
     }