Refactor: Make account 100% optional, default to local-only mode

- SessionManager: return stable local_user ID when not logged in
- Navigation: start directly at ListsNavKey without requiring login
- ListsScreen: add 'Sync / Account' chip in top bar; lists work 100% locally by default
- AuthScreen: update text & add 'Zurück zu meinen Listen' button for returning to local mode
- ADB reinstall & launch verified ✅

android/app/src/main/java/com/example/mitbringsl/Navigation.kt

@@ -1,9 +1,6 @@
 package com.example.mitbringsl
 
-import androidx.compose.foundation.layout.padding
 import androidx.compose.runtime.Composable
-import androidx.compose.runtime.remember
-import androidx.compose.ui.Modifier
 import androidx.navigation3.runtime.entryProvider
 import androidx.navigation3.runtime.rememberNavBackStack
 import androidx.navigation3.ui.NavDisplay
@@ -14,34 +11,30 @@ import com.example.mitbringsl.ui.lists.ListsScreen
 
 @Composable
 fun MainNavigation(sessionManager: SessionManager) {
-    val initialKey = remember {
+    // Local-First: App starts directly at ListsNavKey by default!
-        if (!sessionManager.getToken().isNullOrBlank()) ListsNavKey else AuthNavKey
+    val backStack = rememberNavBackStack(ListsNavKey)
-    }
-    val backStack = rememberNavBackStack(initialKey)
 
     NavDisplay(
         backStack = backStack,
         onBack = { backStack.removeLastOrNull() },
         entryProvider = entryProvider {
-            entry<AuthNavKey> {
-                AuthScreen(
-                    onLoginSuccess = {
-                        backStack.clear()
-                        backStack.add(ListsNavKey)
-                    }
-                )
-            }
             entry<ListsNavKey> {
                 ListsScreen(
                     onSelectList = { listId ->
                         backStack.add(ListDetailNavKey(listId))
                     },
-                    onLogout = {
+                    onOpenSync = {
-                        backStack.clear()
                         backStack.add(AuthNavKey)
                     }
                 )
             }
+            entry<AuthNavKey> {
+                AuthScreen(
+                    onLoginSuccess = {
+                        backStack.removeLastOrNull()
+                    }
+                )
+            }
             entry<ListDetailNavKey> { key ->
                 ListDetailScreen(
                     onBack = { backStack.removeLastOrNull() }

android/app/src/main/java/com/example/mitbringsl/data/auth/SessionManager.kt

@@ -6,11 +6,16 @@ import dagger.hilt.android.qualifiers.ApplicationContext
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asStateFlow
+import java.util.UUID
 import javax.inject.Inject
 import javax.inject.Singleton
 
 /**
- * Manages the user's session state and token storage.
+ * Manages the user's session state and local device identification.
+ *
+ * Local-First design: Users do NOT need an account to use the app locally.
+ * When not logged in, [getUserId] returns a stable local ID ("local_user").
+ * Logging in is 100% optional and only required to sync across devices.
  */
 @Singleton
 class SessionManager @Inject constructor(
@@ -18,12 +23,29 @@ class SessionManager @Inject constructor(
 ) {
     private val prefs = context.getSharedPreferences("session", Context.MODE_PRIVATE)
 
+    companion object {
+        const val LOCAL_USER_ID = "local_user"
+    }
+
     private val _isLoggedIn = MutableStateFlow(!getToken().isNullOrBlank())
     val isLoggedIn: StateFlow<Boolean> = _isLoggedIn.asStateFlow()
 
     fun getToken(): String? = prefs.getString("token", null)
 
-    fun getUserId(): String? = prefs.getString("user_id", null)
+    /**
+     * Returns the authenticated user's ID if logged in, or [LOCAL_USER_ID] for local-only mode.
+     */
+    fun getUserId(): String {
+        val remoteId = prefs.getString("user_id", null)
+        if (!remoteId.isNullOrBlank()) return remoteId
+
+        var localId = prefs.getString("local_device_user_id", null)
+        if (localId.isNullOrBlank()) {
+            localId = LOCAL_USER_ID
+            prefs.edit { putString("local_device_user_id", localId) }
+        }
+        return localId
+    }
 
     fun getUserEmail(): String? = prefs.getString("user_email", null)
 
@@ -37,7 +59,11 @@ class SessionManager @Inject constructor(
     }
 
     fun clearSession() {
-        prefs.edit { clear() }
+        prefs.edit {
+            remove("token")
+            remove("user_id")
+            remove("user_email")
+        }
         _isLoggedIn.value = false
     }
 }

android/app/src/main/java/com/example/mitbringsl/ui/auth/AuthScreen.kt

@@ -16,7 +16,6 @@ import androidx.compose.runtime.getValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Brush
-import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.platform.LocalFocusManager
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.input.ImeAction
@@ -37,11 +36,7 @@ fun AuthScreen(
 ) {
     val uiState by viewModel.uiState.collectAsStateWithLifecycle()
     val isLoggedIn by viewModel.sessionManager.isLoggedIn.collectAsStateWithLifecycle()
-
+    val userEmail = viewModel.sessionManager.getUserEmail()
-    if (isLoggedIn) {
-        onLoginSuccess()
-        return
-    }
 
     val focusManager = LocalFocusManager.current
     val gradientBrush = Brush.verticalGradient(
@@ -74,24 +69,21 @@ fun AuthScreen(
                 shadowElevation = 8.dp
             ) {
                 Box(contentAlignment = Alignment.Center) {
-                    Text(
+                    Text(text = "🛒", fontSize = 36.sp)
-                        text = "🛒",
-                        fontSize = 36.sp
-                    )
                 }
             }
 
             Spacer(modifier = Modifier.height(16.dp))
 
             Text(
-                text = "Mitbringsl",
+                text = "Mitbringsl Sync",
                 style = MaterialTheme.typography.headlineLarge,
                 fontWeight = FontWeight.Bold,
                 color = MaterialTheme.colorScheme.onSurface
             )
 
             Text(
-                text = "Local-First Einkaufsliste — Werbefrei & Sicher",
+                text = "Melde dich an, um deine Listen geräteübergreifend zu synchronisieren.",
                 style = MaterialTheme.typography.bodyMedium,
                 color = MaterialTheme.colorScheme.onSurfaceVariant,
                 textAlign = TextAlign.Center
@@ -112,8 +104,36 @@ fun AuthScreen(
                     modifier = Modifier.padding(24.dp),
                     horizontalAlignment = Alignment.CenterHorizontally
                 ) {
+                    if (isLoggedIn) {
                         Text(
-                        text = if (uiState.isRegisteringMode) "Konto erstellen" else "Willkommen zurück",
+                            text = "Angemeldet als",
+                            style = MaterialTheme.typography.labelMedium,
+                            color = MaterialTheme.colorScheme.onSurfaceVariant
+                        )
+                        Spacer(modifier = Modifier.height(4.dp))
+                        Text(
+                            text = userEmail ?: "Benutzer",
+                            style = MaterialTheme.typography.titleMedium,
+                            fontWeight = FontWeight.Bold,
+                            color = MaterialTheme.colorScheme.onSurface
+                        )
+                        Spacer(modifier = Modifier.height(24.dp))
+
+                        Button(
+                            onClick = {
+                                viewModel.sessionManager.clearSession()
+                            },
+                            colors = ButtonDefaults.buttonColors(
+                                containerColor = MaterialTheme.colorScheme.error
+                            ),
+                            shape = RoundedCornerShape(16.dp),
+                            modifier = Modifier.fillMaxWidth()
+                        ) {
+                            Text("Abmelden")
+                        }
+                    } else {
+                        Text(
+                            text = if (uiState.isRegisteringMode) "Konto erstellen" else "Anmelden",
                             style = MaterialTheme.typography.titleLarge,
                             fontWeight = FontWeight.SemiBold,
                             color = MaterialTheme.colorScheme.onSurface
@@ -226,7 +246,7 @@ fun AuthScreen(
                             }
                         }
 
-                    Spacer(modifier = Modifier.height(16.dp))
+                        Spacer(modifier = Modifier.height(12.dp))
 
                         // Toggle Login / Register mode
                         TextButton(onClick = viewModel::toggleMode) {
@@ -241,6 +261,16 @@ fun AuthScreen(
                             )
                         }
                     }
+
+                    Spacer(modifier = Modifier.height(8.dp))
+
+                    TextButton(onClick = onLoginSuccess) {
+                        Text(
+                            text = "Zurück zu meinen Listen",
+                            color = MaterialTheme.colorScheme.onSurfaceVariant
+                        )
+                    }
+                }
             }
 
             Spacer(modifier = Modifier.height(32.dp))

android/app/src/main/java/com/example/mitbringsl/ui/lists/ListsScreen.kt

@@ -3,7 +3,6 @@ package com.example.mitbringsl.ui.lists
 import androidx.compose.animation.AnimatedVisibility
 import androidx.compose.animation.expandVertically
 import androidx.compose.animation.shrinkVertically
-import androidx.compose.foundation.background
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.lazy.LazyColumn
@@ -11,8 +10,10 @@ import androidx.compose.foundation.lazy.items
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.Add
+import androidx.compose.material.icons.filled.CloudDone
+import androidx.compose.material.icons.filled.CloudOff
 import androidx.compose.material.icons.filled.Delete
-import androidx.compose.material.icons.filled.ExitToApp
+import androidx.compose.material.icons.filled.Person
 import androidx.compose.material.icons.filled.ShoppingCart
 import androidx.compose.material.icons.filled.WifiOff
 import androidx.compose.material3.*
@@ -31,12 +32,15 @@ import com.example.mitbringsl.data.local.entity.ListEntity
 @Composable
 fun ListsScreen(
     onSelectList: (String) -> Unit,
-    onLogout: () -> Unit,
+    onOpenSync: () -> Unit,
     modifier: Modifier = Modifier,
     viewModel: ListsViewModel = hiltViewModel(),
 ) {
     val lists by viewModel.lists.collectAsStateWithLifecycle()
     val isOnline by viewModel.isOnline.collectAsStateWithLifecycle()
+    val isLoggedIn by viewModel.sessionManager.isLoggedIn.collectAsStateWithLifecycle()
+    val userEmail = viewModel.sessionManager.getUserEmail()
+
     var showAddDialog by remember { mutableStateOf(false) }
 
     Scaffold(
@@ -49,15 +53,26 @@ fun ListsScreen(
                     )
                 },
                 actions = {
-                    IconButton(onClick = {
+                    // Sync / Account button
-                        viewModel.logout()
+                    AssistChip(
-                        onLogout()
+                        onClick = onOpenSync,
-                    }) {
+                        label = {
-                        Icon(
+                            Text(
-                            imageVector = Icons.Default.ExitToApp,
+                                text = if (isLoggedIn) (userEmail?.substringBefore("@") ?: "Sync aktiv") else "Sync / Account",
-                            contentDescription = "Abmelden"
+                                style = MaterialTheme.typography.labelSmall
+                            )
+                        },
+                        leadingIcon = {
+                            Icon(
+                                imageVector = if (isLoggedIn) Icons.Default.CloudDone else Icons.Default.CloudOff,
+                                contentDescription = null,
+                                modifier = Modifier.size(16.dp),
+                                tint = if (isLoggedIn) MaterialTheme.colorScheme.primary else MaterialTheme.colorScheme.outline
+                            )
+                        },
+                        shape = RoundedCornerShape(20.dp),
+                        modifier = Modifier.padding(end = 8.dp)
                     )
-                    }
                 },
                 colors = TopAppBarDefaults.topAppBarColors(
                     containerColor = MaterialTheme.colorScheme.surface
@@ -106,7 +121,7 @@ fun ListsScreen(
                         )
                         Spacer(modifier = Modifier.width(8.dp))
                         Text(
-                            text = "Offline – Änderungen werden lokal gespeichert",
+                            text = "Offline – Änderungen bleiben lokal gespeichert",
                             style = MaterialTheme.typography.labelMedium,
                             color = MaterialTheme.colorScheme.onErrorContainer
                         )
@@ -140,7 +155,7 @@ fun ListsScreen(
                         )
                         Spacer(modifier = Modifier.height(8.dp))
                         Text(
-                            text = "Erstelle deine erste Liste mit dem Plus-Button unten rechts.",
+                            text = "Erstelle direkt deine erste Liste — ohne Anmeldung nutzbar!",
                             style = MaterialTheme.typography.bodyMedium,
                             color = MaterialTheme.colorScheme.onSurfaceVariant,
                             textAlign = TextAlign.Center