feat(navigation): implement CRUD navigation and MainActivity integration

ui/navigation/Screen.kt:
- Sealed interface with @Serializable routes: ItemList, ItemForm,
  CategoryManagement, LocationManagement
- ItemForm accepts optional itemId for edit mode (type-safe navigation)

ui/navigation/KrisenvorratNavGraph.kt:
- NavHost with ItemList as start destination
- Screen wiring: ItemList -> ItemForm (create/edit), CategoryManagement,
  LocationManagement with back navigation via popBackStack()

ui/item/ItemListScreen.kt:
- Added onItemClick, onCategoriesClick, onLocationsClick callbacks
- TopAppBar MoreVert dropdown menu for category/location management
- ItemCard now clickable to navigate to edit mode

MainActivity.kt:
- Replaced placeholder with NavHost via KrisenvorratNavGraph
- rememberNavController() as root navigation controller

ui/navigation/ScreenTest.kt:
- 7 tests covering route instantiation, nullable itemId, equality

Closes #28

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

@@ -4,18 +4,13 @@ import android.os.Bundle
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
-import androidx.compose.foundation.layout.Arrangement
-import androidx.compose.foundation.layout.Column
-import androidx.compose.foundation.layout.fillMaxSize
-import androidx.compose.foundation.layout.padding
 import androidx.compose.material3.MaterialTheme
-import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
-import androidx.compose.ui.Alignment
-import androidx.compose.ui.Modifier
 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.theme.KrisenvorratTheme
 
 @AndroidEntryPoint
@@ -25,25 +20,8 @@ class MainActivity : ComponentActivity() {
         enableEdgeToEdge()
         setContent {
             KrisenvorratTheme {
-                Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
+                val navController = rememberNavController()
-                    Column(
+                KrisenvorratNavGraph(navController = navController)
-                        modifier = Modifier
-                            .fillMaxSize()
-                            .padding(innerPadding),
-                        horizontalAlignment = Alignment.CenterHorizontally,
-                        verticalArrangement = Arrangement.Center
-                    ) {
-                        Text(
-                            text = "Krisenvorrat",
-                            style = MaterialTheme.typography.headlineLarge
-                        )
-                        Text(
-                            text = "v${BuildConfig.VERSION_NAME}",
-                            style = MaterialTheme.typography.bodyMedium,
-                            color = MaterialTheme.colorScheme.onSurfaceVariant
-                        )
-                    }
-                }
             }
         }
     }
@@ -53,6 +31,9 @@ class MainActivity : ComponentActivity() {
 @Composable
 fun DefaultPreview() {
     KrisenvorratTheme {
-        Text("Krisenvorrat")
+        Text(
+            text = "Krisenvorrat",
+            style = MaterialTheme.typography.headlineLarge
+        )
     }
 }

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

@@ -1,5 +1,6 @@
 package de.krisenvorrat.app.ui.item
 
+import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -14,8 +15,11 @@ import androidx.compose.foundation.lazy.items
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.Add
 import androidx.compose.material.icons.filled.Delete
+import androidx.compose.material.icons.filled.MoreVert
 import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.Card
+import androidx.compose.material3.DropdownMenu
+import androidx.compose.material3.DropdownMenuItem
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.material3.FloatingActionButton
 import androidx.compose.material3.Icon
@@ -27,6 +31,9 @@ import androidx.compose.material3.TextButton
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.unit.dp
@@ -39,14 +46,45 @@ import java.time.format.DateTimeFormatter
 @Composable
 internal fun ItemListScreen(
     onAddItem: () -> Unit,
+    onItemClick: (String) -> Unit,
+    onCategoriesClick: () -> Unit,
+    onLocationsClick: () -> Unit,
     viewModel: ItemListViewModel = hiltViewModel()
 ) {
     val uiState by viewModel.uiState.collectAsStateWithLifecycle()
+    var isMenuExpanded by remember { mutableStateOf(false) }
 
     Scaffold(
         topBar = {
             TopAppBar(
-                title = { Text("Artikel") }
+                title = { Text("Artikel") },
+                actions = {
+                    IconButton(onClick = { isMenuExpanded = true }) {
+                        Icon(
+                            imageVector = Icons.Default.MoreVert,
+                            contentDescription = "Menü"
+                        )
+                    }
+                    DropdownMenu(
+                        expanded = isMenuExpanded,
+                        onDismissRequest = { isMenuExpanded = false }
+                    ) {
+                        DropdownMenuItem(
+                            text = { Text("Kategorien") },
+                            onClick = {
+                                isMenuExpanded = false
+                                onCategoriesClick()
+                            }
+                        )
+                        DropdownMenuItem(
+                            text = { Text("Lagerorte") },
+                            onClick = {
+                                isMenuExpanded = false
+                                onLocationsClick()
+                            }
+                        )
+                    }
+                }
             )
         },
         floatingActionButton = {
@@ -67,6 +105,7 @@ internal fun ItemListScreen(
         } else {
             ItemList(
                 groupedItems = uiState.groupedItems,
+                onItemClick = onItemClick,
                 onDeleteClick = viewModel::showDeleteDialog,
                 modifier = Modifier
                     .fillMaxSize()
@@ -109,6 +148,7 @@ private fun EmptyState(modifier: Modifier = Modifier) {
 @Composable
 private fun ItemList(
     groupedItems: Map<String, List<ItemUiModel>>,
+    onItemClick: (String) -> Unit,
     onDeleteClick: (ItemUiModel) -> Unit,
     modifier: Modifier = Modifier
 ) {
@@ -123,6 +163,7 @@ private fun ItemList(
             items(items, key = { it.id }) { item ->
                 ItemCard(
                     item = item,
+                    onClick = { onItemClick(item.id) },
                     onDeleteClick = { onDeleteClick(item) }
                 )
             }
@@ -145,12 +186,14 @@ private fun CategoryHeader(categoryName: String) {
 @Composable
 private fun ItemCard(
     item: ItemUiModel,
+    onClick: () -> Unit,
     onDeleteClick: () -> Unit
 ) {
     Card(
         modifier = Modifier
             .fillMaxWidth()
             .padding(horizontal = 16.dp, vertical = 2.dp)
+            .clickable(onClick = onClick)
     ) {
         Row(
             modifier = Modifier

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

@@ -0,0 +1,59 @@
+package de.krisenvorrat.app.ui.navigation
+
+import androidx.compose.runtime.Composable
+import androidx.navigation.NavHostController
+import androidx.navigation.compose.NavHost
+import androidx.navigation.compose.composable
+import de.krisenvorrat.app.ui.category.CategoryListScreen
+import de.krisenvorrat.app.ui.item.ItemFormScreen
+import de.krisenvorrat.app.ui.item.ItemListScreen
+import de.krisenvorrat.app.ui.location.LocationListScreen
+
+@Composable
+internal fun KrisenvorratNavGraph(navController: NavHostController) {
+    NavHost(
+        navController = navController,
+        startDestination = Screen.ItemList
+    ) {
+        composable<Screen.ItemList> {
+            ItemListScreen(
+                onAddItem = {
+                    navController.navigate(Screen.ItemForm())
+                },
+                onItemClick = { itemId ->
+                    navController.navigate(Screen.ItemForm(itemId = itemId))
+                },
+                onCategoriesClick = {
+                    navController.navigate(Screen.CategoryManagement)
+                },
+                onLocationsClick = {
+                    navController.navigate(Screen.LocationManagement)
+                }
+            )
+        }
+
+        composable<Screen.ItemForm> {
+            ItemFormScreen(
+                onNavigateBack = {
+                    navController.popBackStack()
+                }
+            )
+        }
+
+        composable<Screen.CategoryManagement> {
+            CategoryListScreen(
+                onNavigateBack = {
+                    navController.popBackStack()
+                }
+            )
+        }
+
+        composable<Screen.LocationManagement> {
+            LocationListScreen(
+                onNavigateBack = {
+                    navController.popBackStack()
+                }
+            )
+        }
+    }
+}

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

@@ -0,0 +1,19 @@
+package de.krisenvorrat.app.ui.navigation
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+internal sealed interface Screen {
+
+    @Serializable
+    data object ItemList : Screen
+
+    @Serializable
+    data class ItemForm(val itemId: String? = null) : Screen
+
+    @Serializable
+    data object CategoryManagement : Screen
+
+    @Serializable
+    data object LocationManagement : Screen
+}

app/src/test/java/de/krisenvorrat/app/ui/navigation/ScreenTest.kt

@@ -0,0 +1,76 @@
+package de.krisenvorrat.app.ui.navigation
+
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class ScreenTest {
+
+    @Test
+    fun test_itemListRoute_isSingleton() {
+        // Given & When
+        val route = Screen.ItemList
+
+        // Then
+        assertEquals(Screen.ItemList, route)
+    }
+
+    @Test
+    fun test_itemFormRoute_withoutItemId_hasNullItemId() {
+        // Given & When
+        val route = Screen.ItemForm()
+
+        // Then
+        assertNull(route.itemId)
+    }
+
+    @Test
+    fun test_itemFormRoute_withItemId_hasCorrectItemId() {
+        // Given
+        val expectedId = "test-item-123"
+
+        // When
+        val route = Screen.ItemForm(itemId = expectedId)
+
+        // Then
+        assertEquals(expectedId, route.itemId)
+    }
+
+    @Test
+    fun test_categoryManagementRoute_isSingleton() {
+        // Given & When
+        val route = Screen.CategoryManagement
+
+        // Then
+        assertEquals(Screen.CategoryManagement, route)
+    }
+
+    @Test
+    fun test_locationManagementRoute_isSingleton() {
+        // Given & When
+        val route = Screen.LocationManagement
+
+        // Then
+        assertEquals(Screen.LocationManagement, route)
+    }
+
+    @Test
+    fun test_itemFormRoute_equality_sameItemId_areEqual() {
+        // Given
+        val route1 = Screen.ItemForm(itemId = "abc")
+        val route2 = Screen.ItemForm(itemId = "abc")
+
+        // Then
+        assertEquals(route1, route2)
+    }
+
+    @Test
+    fun test_itemFormRoute_equality_differentItemId_areNotEqual() {
+        // Given
+        val route1 = Screen.ItemForm(itemId = "abc")
+        val route2 = Screen.ItemForm(itemId = "xyz")
+
+        // Then
+        assert(route1 != route2)
+    }
+}