Feature: Shared lists & Invite Code invitation mechanism

- Backend:
  - Migration 000002: add invite_code column to lists & list_members auto-population
  - ListStore: GetLists/GetList check owner_id & list_members; add GetInviteCode & JoinByInviteCode
  - HTTP API: add POST /api/lists/{id}/invite & POST /api/lists/join
- Android App:
  - DTOs & MitbringslApi: add JoinListRequestDto, InviteCodeResponseDto & endpoints
  - ListsScreen & ViewModel: add 'Liste beitreten' action button & dialog for entering invite code
  - ListDetailScreen & ViewModel: add 'Liste teilen' action icon in top bar with System Share Sheet
- ADB reinstall & launch verified ✅

android/app/src/main/java/com/example/mitbringsl/data/remote/api/MitbringslApi.kt

@@ -6,10 +6,6 @@ import retrofit2.http.*
 
 /**
  * Retrofit API interface matching the mitbringsl backend.
- *
- * Authentication: every authenticated call must include
- *   Authorization: Bearer <session-token>
- * This is injected by the AuthInterceptor in the OkHttp client.
  */
 interface MitbringslApi {
 
@@ -35,9 +31,15 @@ interface MitbringslApi {
     @POST("api/lists")
     suspend fun createList(@Body body: CreateListRequestDto): Response<ListDto>
 
+    @POST("api/lists/join")
+    suspend fun joinList(@Body body: JoinListRequestDto): Response<ListDto>
+
     @GET("api/lists/{id}")
     suspend fun getList(@Path("id") listId: String): Response<ListDetailDto>
 
+    @POST("api/lists/{id}/invite")
+    suspend fun getInviteCode(@Path("id") listId: String): Response<InviteCodeResponseDto>
+
     // --- Ops ----------------------------------------------------------------
 
     @POST("api/lists/{id}/ops")

android/app/src/main/java/com/example/mitbringsl/data/remote/dto/Dtos.kt

@@ -48,10 +48,21 @@ data class UserDto(
 @Serializable
 data class CreateListRequestDto(val name: String)
 
+@Serializable
+data class JoinListRequestDto(
+    @SerialName("invite_code") val inviteCode: String
+)
+
+@Serializable
+data class InviteCodeResponseDto(
+    @SerialName("invite_code") val inviteCode: String
+)
+
 @Serializable
 data class ListDto(
     val id: String,
     val name: String,
+    @SerialName("invite_code") val inviteCode: String? = null,
     @SerialName("updated_at") val updatedAt: String,
     @SerialName("hlc_ts") val hlcTs: Long,
 )
@@ -63,6 +74,7 @@ data class ListsResponseDto(val lists: List<ListDto>)
 data class ListDetailDto(
     val id: String,
     val name: String,
+    @SerialName("invite_code") val inviteCode: String? = null,
     @SerialName("updated_at") val updatedAt: String,
     @SerialName("hlc_ts") val hlcTs: Long,
     val items: List<ItemDto>,

android/app/src/main/java/com/example/mitbringsl/ui/detail/ListDetailScreen.kt

@@ -1,5 +1,6 @@
 package com.example.mitbringsl.ui.detail
 
+import android.content.Intent
 import androidx.compose.animation.animateColorAsState
 import androidx.compose.animation.core.animateFloatAsState
 import androidx.compose.foundation.background
@@ -12,13 +13,15 @@ import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.filled.ArrowBack
 import androidx.compose.material.icons.filled.Add
 import androidx.compose.material.icons.filled.Close
+import androidx.compose.material.icons.filled.Share
 import androidx.compose.material3.*
 import androidx.compose.runtime.*
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.alpha
-import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.text.style.TextDecoration
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
@@ -41,6 +44,10 @@ fun ListDetailScreen(
     val items by viewModel.items.collectAsStateWithLifecycle()
     val query by viewModel.query.collectAsStateWithLifecycle()
     val suggestions by viewModel.suggestions.collectAsStateWithLifecycle()
+    val inviteCode by viewModel.inviteCode.collectAsStateWithLifecycle()
+
+    val context = LocalContext.current
+    var showShareDialog by remember { mutableStateOf(false) }
 
     val (openItems, checkedItems) = remember(items) {
         items.partition { !it.checked }
@@ -60,6 +67,16 @@ fun ListDetailScreen(
                         )
                     }
                 },
+                actions = {
+                    if (!inviteCode.isNullOrBlank()) {
+                        IconButton(onClick = { showShareDialog = true }) {
+                            Icon(
+                                imageVector = Icons.Default.Share,
+                                contentDescription = "Liste teilen"
+                            )
+                        }
+                    }
+                },
                 colors = TopAppBarDefaults.topAppBarColors(
                     containerColor = MaterialTheme.colorScheme.surface
                 )
@@ -229,6 +246,65 @@ fun ListDetailScreen(
             }
         }
     }
+
+    if (showShareDialog && !inviteCode.isNullOrBlank()) {
+        AlertDialog(
+            onDismissRequest = { showShareDialog = false },
+            title = { Text("Einkaufsliste teilen") },
+            text = {
+                Column(horizontalAlignment = Alignment.CenterHorizontally) {
+                    Text(
+                        text = "Gib diesen Einladungs-Code an andere Personen weiter:",
+                        style = MaterialTheme.typography.bodySmall,
+                        color = MaterialTheme.colorScheme.onSurfaceVariant
+                    )
+                    Spacer(modifier = Modifier.height(16.dp))
+                    Surface(
+                        shape = RoundedCornerShape(12.dp),
+                        color = MaterialTheme.colorScheme.primaryContainer,
+                        modifier = Modifier.fillMaxWidth()
+                    ) {
+                        Text(
+                            text = inviteCode!!,
+                            style = MaterialTheme.typography.headlineMedium,
+                            fontWeight = FontWeight.Bold,
+                            color = MaterialTheme.colorScheme.onPrimaryContainer,
+                            textAlign = TextAlign.Center,
+                            modifier = Modifier.padding(16.dp)
+                        )
+                    }
+                }
+            },
+            confirmButton = {
+                Button(
+                    onClick = {
+                        val sendIntent = Intent().apply {
+                            action = Intent.ACTION_SEND
+                            putExtra(
+                                Intent.EXTRA_TEXT,
+                                "Tritt meiner Einkaufsliste bei Mitbringsl bei! Einladungs-Code: ${inviteCode!!}"
+                            )
+                            type = "text/plain"
+                        }
+                        val shareIntent = Intent.createChooser(sendIntent, "Einladungs-Code teilen")
+                        context.startActivity(shareIntent)
+                        showShareDialog = false
+                    },
+                    shape = RoundedCornerShape(12.dp)
+                ) {
+                    Icon(imageVector = Icons.Default.Share, contentDescription = null)
+                    Spacer(modifier = Modifier.width(8.dp))
+                    Text("Code teilen")
+                }
+            },
+            dismissButton = {
+                TextButton(onClick = { showShareDialog = false }) {
+                    Text("Schließen")
+                }
+            },
+            shape = RoundedCornerShape(24.dp)
+        )
+    }
 }
 
 @Composable

android/app/src/main/java/com/example/mitbringsl/ui/detail/ListDetailViewModel.kt

@@ -30,9 +30,26 @@ class ListDetailViewModel @Inject constructor(
             initialValue = emptyList()
         )
 
+    private val _inviteCode = MutableStateFlow<String?>(null)
+    val inviteCode: StateFlow<String?> = _inviteCode.asStateFlow()
+
     fun setListId(id: String) {
         if (_listId.value != id) {
             _listId.value = id
+            fetchInviteCode(id)
+        }
+    }
+
+    private fun fetchInviteCode(id: String) {
+        viewModelScope.launch {
+            try {
+                val resp = api.getInviteCode(id)
+                if (resp.isSuccessful && resp.body() != null) {
+                    _inviteCode.value = resp.body()!!.inviteCode
+                }
+            } catch (_: Exception) {
+                // Offline or local list
+            }
         }
     }
 

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

@@ -13,7 +13,7 @@ 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.Person
+import androidx.compose.material.icons.filled.GroupAdd
 import androidx.compose.material.icons.filled.ShoppingCart
 import androidx.compose.material.icons.filled.WifiOff
 import androidx.compose.material3.*
@@ -40,8 +40,10 @@ fun ListsScreen(
     val isOnline by viewModel.isOnline.collectAsStateWithLifecycle()
     val isLoggedIn by viewModel.sessionManager.isLoggedIn.collectAsStateWithLifecycle()
     val userEmail = viewModel.sessionManager.getUserEmail()
+    val joinError by viewModel.joinError.collectAsStateWithLifecycle()
 
     var showAddDialog by remember { mutableStateOf(false) }
+    var showJoinDialog by remember { mutableStateOf(false) }
 
     Scaffold(
         topBar = {
@@ -53,7 +55,13 @@ fun ListsScreen(
                     )
                 },
                 actions = {
-                    // Sync / Account button
+                    IconButton(onClick = { showJoinDialog = true }) {
+                        Icon(
+                            imageVector = Icons.Default.GroupAdd,
+                            contentDescription = "Liste beitreten"
+                        )
+                    }
+
                     AssistChip(
                         onClick = onOpenSync,
                         label = {
@@ -155,11 +163,20 @@ fun ListsScreen(
                         )
                         Spacer(modifier = Modifier.height(8.dp))
                         Text(
-                            text = "Erstelle direkt deine erste Liste — ohne Anmeldung nutzbar!",
+                            text = "Erstelle deine erste Liste oder tritt einer bestehenden Liste per Einladungs-Code bei.",
                             style = MaterialTheme.typography.bodyMedium,
                             color = MaterialTheme.colorScheme.onSurfaceVariant,
                             textAlign = TextAlign.Center
                         )
+                        Spacer(modifier = Modifier.height(16.dp))
+                        OutlinedButton(
+                            onClick = { showJoinDialog = true },
+                            shape = RoundedCornerShape(12.dp)
+                        ) {
+                            Icon(imageVector = Icons.Default.GroupAdd, contentDescription = null)
+                            Spacer(modifier = Modifier.width(8.dp))
+                            Text("Liste per Code beitreten")
+                        }
                     }
                 } else {
                     LazyColumn(
@@ -189,6 +206,21 @@ fun ListsScreen(
             }
         )
     }
+
+    if (showJoinDialog) {
+        JoinListDialog(
+            errorMessage = joinError,
+            onDismiss = {
+                viewModel.clearJoinError()
+                showJoinDialog = false
+            },
+            onConfirm = { code ->
+                viewModel.joinList(code, onSuccess = {
+                    showJoinDialog = false
+                })
+            }
+        )
+    }
 }
 
 @Composable
@@ -287,3 +319,59 @@ fun CreateListDialog(
         shape = RoundedCornerShape(24.dp)
     )
 }
+
+@Composable
+fun JoinListDialog(
+    errorMessage: String?,
+    onDismiss: () -> Unit,
+    onConfirm: (String) -> Unit
+) {
+    var code by remember { mutableStateOf("") }
+
+    AlertDialog(
+        onDismissRequest = onDismiss,
+        title = { Text("Liste beitreten") },
+        text = {
+            Column {
+                Text(
+                    text = "Gib den 8-stelligen Einladungs-Code ein, um einer geteilten Liste beizutreten:",
+                    style = MaterialTheme.typography.bodySmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant
+                )
+                Spacer(modifier = Modifier.height(12.dp))
+                OutlinedTextField(
+                    value = code,
+                    onValueChange = { code = it.uppercase() },
+                    label = { Text("Einladungs-Code") },
+                    placeholder = { Text("z.B. X7K9P2A1") },
+                    singleLine = true,
+                    shape = RoundedCornerShape(12.dp),
+                    modifier = Modifier.fillMaxWidth()
+                )
+                if (errorMessage != null) {
+                    Spacer(modifier = Modifier.height(8.dp))
+                    Text(
+                        text = errorMessage,
+                        style = MaterialTheme.typography.bodySmall,
+                        color = MaterialTheme.colorScheme.error
+                    )
+                }
+            }
+        },
+        confirmButton = {
+            Button(
+                onClick = { onConfirm(code) },
+                enabled = code.isNotBlank(),
+                shape = RoundedCornerShape(12.dp)
+            ) {
+                Text("Beitreten")
+            }
+        },
+        dismissButton = {
+            TextButton(onClick = onDismiss) {
+                Text("Abbrechen")
+            }
+        },
+        shape = RoundedCornerShape(24.dp)
+    )
+}

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

@@ -3,12 +3,17 @@ package com.example.mitbringsl.ui.lists
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import com.example.mitbringsl.data.auth.SessionManager
+import com.example.mitbringsl.data.local.dao.ListDao
 import com.example.mitbringsl.data.local.entity.ListEntity
+import com.example.mitbringsl.data.remote.api.MitbringslApi
+import com.example.mitbringsl.data.remote.dto.JoinListRequestDto
 import com.example.mitbringsl.data.repository.ShoppingRepository
 import com.example.mitbringsl.util.NetworkMonitor
 import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
 import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
 import javax.inject.Inject
@@ -16,6 +21,8 @@ import javax.inject.Inject
 @HiltViewModel
 class ListsViewModel @Inject constructor(
     private val shoppingRepository: ShoppingRepository,
+    private val listDao: ListDao,
+    private val api: MitbringslApi,
     val sessionManager: SessionManager,
     networkMonitor: NetworkMonitor,
 ) : ViewModel() {
@@ -34,6 +41,9 @@ class ListsViewModel @Inject constructor(
             initialValue = emptyList()
         )
 
+    private val _joinError = MutableStateFlow<String?>(null)
+    val joinError: StateFlow<String?> = _joinError.asStateFlow()
+
     fun createList(name: String) {
         if (name.isBlank()) return
         viewModelScope.launch {
@@ -41,6 +51,39 @@ class ListsViewModel @Inject constructor(
         }
     }
 
+    fun joinList(inviteCode: String, onSuccess: () -> Unit) {
+        if (inviteCode.isBlank()) return
+        viewModelScope.launch {
+            _joinError.value = null
+            try {
+                val resp = api.joinList(JoinListRequestDto(inviteCode.trim()))
+                if (resp.isSuccessful && resp.body() != null) {
+                    val dto = resp.body()!!
+                    val ownerId = sessionManager.getUserId()
+                    listDao.upsert(
+                        ListEntity(
+                            id = dto.id,
+                            name = dto.name,
+                            ownerId = ownerId,
+                            createdAt = System.currentTimeMillis(),
+                            updatedAt = System.currentTimeMillis(),
+                            hlcTs = dto.hlcTs
+                        )
+                    )
+                    onSuccess()
+                } else {
+                    _joinError.value = "Ungültiger Einladungs-Code oder Serverfehler."
+                }
+            } catch (e: Exception) {
+                _joinError.value = e.localizedMessage ?: "Netzwerkfehler beim Beitreten."
+            }
+        }
+    }
+
+    fun clearJoinError() {
+        _joinError.value = null
+    }
+
     fun deleteList(listId: String) {
         viewModelScope.launch {
             shoppingRepository.deleteList(listId)