Initial setup for navigation display with nav3

Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>

shared/src/commonMain/kotlin/net/newpipe/app/App.kt

@@ -6,15 +6,18 @@
 package net.newpipe.app
 
 import androidx.compose.runtime.Composable
-import androidx.compose.ui.tooling.preview.Preview
 import net.newpipe.app.di.KoinApp
+import net.newpipe.app.navigation.Screen
 import net.newpipe.app.theme.AppTheme
 import org.koin.compose.KoinApplication
 import org.koin.plugin.module.dsl.koinConfiguration
 
+/**
+ * Entry point for the multiplatform compose application
+ * @param startDestination Starting destination for the app
+ */
 @Composable
-@Preview
-fun App() {
+fun App(startDestination: Screen? = null) {
     KoinApplication(configuration = koinConfiguration<KoinApp>()) {
         AppTheme {
         }

shared/src/commonMain/kotlin/net/newpipe/app/navigation/NavDisplay.kt

@@ -0,0 +1,26 @@
+/*
+ * SPDX-FileCopyrightText: 2026 NewPipe e.V. <https://newpipe-ev.de>
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+package net.newpipe.app.navigation
+
+import androidx.compose.runtime.Composable
+import androidx.navigation3.runtime.entryProvider
+import androidx.navigation3.runtime.rememberNavBackStack
+import androidx.navigation3.ui.NavDisplay
+
+/**
+ * Navigation display for compose screens
+ * @param startDestination Starting destination for the app
+ */
+@Composable
+fun NavDisplay(startDestination: Screen) {
+    val backstack = rememberNavBackStack(screenConfig, startDestination)
+
+    NavDisplay(
+        backStack = backstack,
+        entryProvider = entryProvider {
+        }
+    )
+}

shared/src/commonMain/kotlin/net/newpipe/app/navigation/Screen.kt

@@ -0,0 +1,31 @@
+/*
+ * SPDX-FileCopyrightText: 2026 NewPipe e.V. <https://newpipe-ev.de>
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+package net.newpipe.app.navigation
+
+import androidx.navigation3.runtime.NavKey
+import androidx.savedstate.serialization.SavedStateConfiguration
+import kotlinx.serialization.ExperimentalSerializationApi
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.modules.SerializersModule
+import kotlinx.serialization.modules.polymorphic
+
+/**
+ * Destinations for navigation in compose
+ */
+@Serializable
+sealed interface Screen : NavKey
+
+/**
+ * Saved state configuration for screens
+ */
+@OptIn(ExperimentalSerializationApi::class)
+internal val screenConfig = SavedStateConfiguration {
+    serializersModule = SerializersModule {
+        polymorphic(NavKey::class) {
+            subclassesOfSealed<Screen>()
+        }
+    }
+}