feat(navigation): implement Bottom Navigation Bar with 4 tabs and app shell

MainScreen.kt: new app shell with Scaffold + Material 3 NavigationBar
providing 4 tabs (Uebersicht, Inventur, Warnungen, Einstellungen).

TopLevelDestination.kt: enum defining tab routes, icons (Home, Inventory2,
Warning, Settings), and labels for the navigation bar.

Screen.kt: added Warnings and Settings sealed interface members.

KrisenvorratNavGraph.kt: accepts Modifier, added Warnings/Settings
composables, removed obsolete DashboardScreen navigation callback.

DashboardScreen.kt: removed Scaffold wrapper and onNavigateToItems param,
now uses Column layout (TopAppBar handled inline).

ItemListScreen.kt: removed onDashboardClick param and Dashboard menu entry
(no longer needed with tab navigation).

WarningsScreen.kt, SettingsScreen.kt: placeholder screens for future impl.

MainActivity.kt: delegates to MainScreen instead of NavGraph directly.

Added material-icons-extended dependency for Inventory2 icon.

Closes #33

app/build.gradle.kts

@@ -53,6 +53,7 @@ dependencies {
     implementation(libs.androidx.ui.graphics)
     implementation(libs.androidx.ui.tooling.preview)
     implementation(libs.androidx.material3)
+    implementation(libs.androidx.material.icons.extended)
 
     // Hilt
     implementation(libs.hilt.android)

app/src/main/java/de/krisenvorrat/app/MainActivity.kt

@@ -8,9 +8,8 @@ import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.tooling.preview.Preview
-import androidx.navigation.compose.rememberNavController
 import dagger.hilt.android.AndroidEntryPoint
-import de.krisenvorrat.app.ui.navigation.KrisenvorratNavGraph
+import de.krisenvorrat.app.ui.MainScreen
 import de.krisenvorrat.app.ui.theme.KrisenvorratTheme
 
 @AndroidEntryPoint
@@ -20,8 +19,7 @@ class MainActivity : ComponentActivity() {
         enableEdgeToEdge()
         setContent {
             KrisenvorratTheme {
-                val navController = rememberNavController()
-                KrisenvorratNavGraph(navController = navController)
+                MainScreen()
             }
         }
     }

app/src/main/java/de/krisenvorrat/app/ui/MainScreen.kt

@@ -0,0 +1,72 @@
+package de.krisenvorrat.app.ui
+
+import androidx.compose.foundation.layout.consumeWindowInsets
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.Icon
+import androidx.compose.material3.NavigationBar
+import androidx.compose.material3.NavigationBarItem
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Modifier
+import androidx.navigation.NavDestination.Companion.hasRoute
+import androidx.navigation.NavGraph.Companion.findStartDestination
+import androidx.navigation.compose.currentBackStackEntryAsState
+import androidx.navigation.compose.rememberNavController
+import de.krisenvorrat.app.ui.navigation.KrisenvorratNavGraph
+import de.krisenvorrat.app.ui.navigation.TopLevelDestination
+
+@Composable
+internal fun MainScreen() {
+    val navController = rememberNavController()
+    val navBackStackEntry by navController.currentBackStackEntryAsState()
+    val currentDestination = navBackStackEntry?.destination
+
+    val showBottomBar = TopLevelDestination.entries.any { topLevel ->
+        currentDestination?.hasRoute(topLevel.route::class) == true
+    }
+
+    Scaffold(
+        bottomBar = {
+            if (showBottomBar) {
+                NavigationBar {
+                    TopLevelDestination.entries.forEach { destination ->
+                        val isSelected = currentDestination?.hasRoute(destination.route::class) == true
+
+                        NavigationBarItem(
+                            selected = isSelected,
+                            onClick = {
+                                navController.navigate(destination.route) {
+                                    popUpTo(navController.graph.findStartDestination().id) {
+                                        saveState = true
+                                    }
+                                    launchSingleTop = true
+                                    restoreState = true
+                                }
+                            },
+                            icon = {
+                                Icon(
+                                    imageVector = if (isSelected) {
+                                        destination.selectedIcon
+                                    } else {
+                                        destination.unselectedIcon
+                                    },
+                                    contentDescription = destination.label
+                                )
+                            },
+                            label = { Text(destination.label) }
+                        )
+                    }
+                }
+            }
+        }
+    ) { innerPadding ->
+        KrisenvorratNavGraph(
+            navController = navController,
+            modifier = Modifier
+                .padding(innerPadding)
+                .consumeWindowInsets(innerPadding)
+        )
+    }
+}

app/src/main/java/de/krisenvorrat/app/ui/dashboard/DashboardScreen.kt

@@ -10,16 +10,11 @@ import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.items
-import androidx.compose.material.icons.Icons
-import androidx.compose.material.icons.filled.List
 import androidx.compose.material3.Card
 import androidx.compose.material3.CardDefaults
 import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.ExperimentalMaterial3Api
-import androidx.compose.material3.Icon
-import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
-import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
@@ -42,38 +37,23 @@ import kotlin.math.roundToInt
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
 internal fun DashboardScreen(
-    onNavigateToItems: () -> Unit,
     viewModel: DashboardViewModel = hiltViewModel()
 ) {
     val uiState by viewModel.uiState.collectAsStateWithLifecycle()
 
-    Scaffold(
-        topBar = {
-            TopAppBar(
-                title = { Text("Dashboard") },
-                actions = {
-                    IconButton(onClick = onNavigateToItems) {
-                        Icon(
-                            imageVector = Icons.Default.List,
-                            contentDescription = "Artikelliste"
-                        )
-                    }
-                }
-            )
-        }
-    ) { padding ->
+    Column(modifier = Modifier.fillMaxSize()) {
+        TopAppBar(title = { Text("Übersicht") })
+
         if (uiState.isLoading) {
             CircularProgressIndicator(
                 modifier = Modifier
                     .fillMaxSize()
-                    .padding(padding)
                     .padding(16.dp)
             )
         } else {
             LazyColumn(
                 modifier = Modifier
                     .fillMaxSize()
-                    .padding(padding)
                     .padding(horizontal = 16.dp),
                 verticalArrangement = Arrangement.spacedBy(12.dp)
             ) {

app/src/main/java/de/krisenvorrat/app/ui/item/ItemListScreen.kt

@@ -49,7 +49,6 @@ internal fun ItemListScreen(
     onItemClick: (String) -> Unit,
     onCategoriesClick: () -> Unit,
     onLocationsClick: () -> Unit,
-    onDashboardClick: () -> Unit,
     viewModel: ItemListViewModel = hiltViewModel()
 ) {
     val uiState by viewModel.uiState.collectAsStateWithLifecycle()
@@ -70,13 +69,6 @@ internal fun ItemListScreen(
                         expanded = isMenuExpanded,
                         onDismissRequest = { isMenuExpanded = false }
                     ) {
-                        DropdownMenuItem(
-                            text = { Text("Dashboard") },
-                            onClick = {
-                                isMenuExpanded = false
-                                onDashboardClick()
-                            }
-                        )
                         DropdownMenuItem(
                             text = { Text("Kategorien") },
                             onClick = {

app/src/main/java/de/krisenvorrat/app/ui/navigation/KrisenvorratNavGraph.kt

@@ -1,6 +1,7 @@
 package de.krisenvorrat.app.ui.navigation
 
 import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
 import androidx.navigation.NavHostController
 import androidx.navigation.compose.NavHost
 import androidx.navigation.compose.composable
@@ -9,19 +10,21 @@ import de.krisenvorrat.app.ui.dashboard.DashboardScreen
 import de.krisenvorrat.app.ui.item.ItemFormScreen
 import de.krisenvorrat.app.ui.item.ItemListScreen
 import de.krisenvorrat.app.ui.location.LocationListScreen
+import de.krisenvorrat.app.ui.settings.SettingsScreen
+import de.krisenvorrat.app.ui.warnings.WarningsScreen
 
 @Composable
-internal fun KrisenvorratNavGraph(navController: NavHostController) {
+internal fun KrisenvorratNavGraph(
+    navController: NavHostController,
+    modifier: Modifier = Modifier
+) {
     NavHost(
         navController = navController,
-        startDestination = Screen.Dashboard
+        startDestination = Screen.Dashboard,
+        modifier = modifier
     ) {
         composable<Screen.Dashboard> {
-            DashboardScreen(
-                onNavigateToItems = {
-                    navController.navigate(Screen.ItemList)
-                }
-            )
+            DashboardScreen()
         }
 
         composable<Screen.ItemList> {
@@ -37,11 +40,6 @@ internal fun KrisenvorratNavGraph(navController: NavHostController) {
                 },
                 onLocationsClick = {
                     navController.navigate(Screen.LocationManagement)
-                },
-                onDashboardClick = {
-                    navController.navigate(Screen.Dashboard) {
-                        popUpTo(Screen.Dashboard) { inclusive = true }
-                    }
                 }
             )
         }
@@ -69,5 +67,13 @@ internal fun KrisenvorratNavGraph(navController: NavHostController) {
                 }
             )
         }
+
+        composable<Screen.Warnings> {
+            WarningsScreen()
+        }
+
+        composable<Screen.Settings> {
+            SettingsScreen()
+        }
     }
 }

app/src/main/java/de/krisenvorrat/app/ui/navigation/Screen.kt

@@ -19,4 +19,10 @@ internal sealed interface Screen {
 
     @Serializable
     data object LocationManagement : Screen
+
+    @Serializable
+    data object Warnings : Screen
+
+    @Serializable
+    data object Settings : Screen
 }

app/src/main/java/de/krisenvorrat/app/ui/navigation/TopLevelDestination.kt

@@ -0,0 +1,43 @@
+package de.krisenvorrat.app.ui.navigation
+
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.filled.Home
+import androidx.compose.material.icons.filled.Settings
+import androidx.compose.material.icons.filled.Warning
+import androidx.compose.material.icons.outlined.Home
+import androidx.compose.material.icons.outlined.Inventory2
+import androidx.compose.material.icons.outlined.Settings
+import androidx.compose.material.icons.outlined.Warning
+import androidx.compose.ui.graphics.vector.ImageVector
+
+internal enum class TopLevelDestination(
+    val route: Screen,
+    val selectedIcon: ImageVector,
+    val unselectedIcon: ImageVector,
+    val label: String
+) {
+    OVERVIEW(
+        route = Screen.Dashboard,
+        selectedIcon = Icons.Filled.Home,
+        unselectedIcon = Icons.Outlined.Home,
+        label = "Übersicht"
+    ),
+    INVENTORY(
+        route = Screen.ItemList,
+        selectedIcon = Icons.Outlined.Inventory2,
+        unselectedIcon = Icons.Outlined.Inventory2,
+        label = "Inventur"
+    ),
+    WARNINGS(
+        route = Screen.Warnings,
+        selectedIcon = Icons.Filled.Warning,
+        unselectedIcon = Icons.Outlined.Warning,
+        label = "Warnungen"
+    ),
+    SETTINGS(
+        route = Screen.Settings,
+        selectedIcon = Icons.Filled.Settings,
+        unselectedIcon = Icons.Outlined.Settings,
+        label = "Einstellungen"
+    );
+}

app/src/main/java/de/krisenvorrat/app/ui/settings/SettingsScreen.kt

@@ -0,0 +1,34 @@
+package de.krisenvorrat.app.ui.settings
+
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+internal fun SettingsScreen() {
+    Column(modifier = Modifier.fillMaxSize()) {
+        TopAppBar(title = { Text("Einstellungen") })
+        Box(
+            modifier = Modifier
+                .fillMaxSize()
+                .padding(16.dp),
+            contentAlignment = Alignment.Center
+        ) {
+            Text(
+                text = "Einstellungen werden in einem späteren Schritt implementiert.",
+                style = MaterialTheme.typography.bodyLarge,
+                color = MaterialTheme.colorScheme.onSurfaceVariant
+            )
+        }
+    }
+}

app/src/main/java/de/krisenvorrat/app/ui/warnings/WarningsScreen.kt

@@ -0,0 +1,34 @@
+package de.krisenvorrat.app.ui.warnings
+
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+internal fun WarningsScreen() {
+    Column(modifier = Modifier.fillMaxSize()) {
+        TopAppBar(title = { Text("Warnungen") })
+        Box(
+            modifier = Modifier
+                .fillMaxSize()
+                .padding(16.dp),
+            contentAlignment = Alignment.Center
+        ) {
+            Text(
+                text = "Warnungen werden in einem späteren Schritt implementiert.",
+                style = MaterialTheme.typography.bodyLarge,
+                color = MaterialTheme.colorScheme.onSurfaceVariant
+            )
+        }
+    }
+}

gradle/libs.versions.toml

@@ -32,6 +32,7 @@ androidx-ui-tooling-preview = { group = "androidx.compose.ui", name = "ui-toolin
 androidx-ui-test-manifest = { group = "androidx.compose.ui", name = "ui-test-manifest" }
 androidx-ui-test-junit4 = { group = "androidx.compose.ui", name = "ui-test-junit4" }
 androidx-material3 = { group = "androidx.compose.material3", name = "material3" }
+androidx-material-icons-extended = { group = "androidx.compose.material", name = "material-icons-extended" }
 hilt-android = { group = "com.google.dagger", name = "hilt-android", version.ref = "hilt" }
 hilt-android-compiler = { group = "com.google.dagger", name = "hilt-android-compiler", version.ref = "hilt" }
 androidx-hilt-navigation-compose = { group = "androidx.hilt", name = "hilt-navigation-compose", version.ref = "hiltNavigationCompose" }