Add PDF invoice generation: implement backend API, introduce PdfService, update frontend repository and UI with download logic, and mark roadmap task complete.

frontend/core/network/src/commonMain/kotlin/at/mocode/frontend/core/network/ApiRoutes.kt

@@ -49,5 +49,6 @@ object ApiRoutes {
     fun buche(kontoId: String) = "$ROOT/konten/$kontoId/buche"
     fun veranstaltungKonten(veranstaltungId: String) = "$ROOT/veranstaltungen/$veranstaltungId/konten"
     fun personKonto(veranstaltungId: String, personId: String) = "$ROOT/veranstaltungen/$veranstaltungId/personen/$personId"
+    fun rechnung(kontoId: String) = "$ROOT/konten/$kontoId/rechnung"
   }
 }

frontend/features/billing-feature/src/commonMain/kotlin/at/mocode/frontend/features/billing/data/DefaultBillingRepository.kt

@@ -38,4 +38,8 @@ class DefaultBillingRepository(
             setBody(request)
         }.body()
     }
+
+    override suspend fun getRechnungPdf(kontoId: String): Result<ByteArray> = runCatching {
+        client.get(ApiRoutes.Billing.rechnung(kontoId)).body()
+    }
 }

frontend/features/billing-feature/src/commonMain/kotlin/at/mocode/frontend/features/billing/data/FakeBillingRepository.kt

@@ -57,4 +57,8 @@ class FakeBillingRepository : BillingRepository {
         konten[index] = updatedKonto
         return Result.success(updatedKonto)
     }
+
+    override suspend fun getRechnungPdf(kontoId: String): Result<ByteArray> {
+        return Result.success("MOCK PDF CONTENT".encodeToByteArray())
+    }
 }

frontend/features/billing-feature/src/commonMain/kotlin/at/mocode/frontend/features/billing/domain/BillingRepository.kt

@@ -31,4 +31,11 @@ interface BillingRepository {
         kontoId: String,
         request: BuchungRequest
     ): Result<TeilnehmerKontoDto>
+
+    /**
+     * Holt das PDF für eine Rechnung.
+     */
+    suspend fun getRechnungPdf(
+        kontoId: String
+    ): Result<ByteArray>
 }

frontend/features/billing-feature/src/commonMain/kotlin/at/mocode/frontend/features/billing/presentation/BillingScreen.kt

@@ -5,10 +5,7 @@ import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.lazy.LazyColumn
 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.Person
-import androidx.compose.material.icons.filled.Receipt
-import androidx.compose.material.icons.filled.Refresh
+import androidx.compose.material.icons.filled.*
 import androidx.compose.material3.*
 import androidx.compose.runtime.*
 import androidx.compose.ui.Alignment
@@ -85,6 +82,16 @@ fun BillingScreen(
             Text("Buchungen", fontWeight = FontWeight.Bold, fontSize = 16.sp)
             Spacer(Modifier.weight(1f))
             if (state.selectedKonto != null) {
+              OutlinedButton(
+                onClick = { viewModel.downloadRechnung() },
+                contentPadding = PaddingValues(horizontal = 8.dp, vertical = 4.dp),
+                modifier = Modifier.height(32.dp)
+              ) {
+                Icon(Icons.Default.PictureAsPdf, contentDescription = null, modifier = Modifier.size(16.dp))
+                Spacer(Modifier.width(4.dp))
+                Text("Rechnung", fontSize = 12.sp)
+              }
+              Spacer(Modifier.width(8.dp))
               Button(
                 onClick = { showBuchungsDialog = true },
                 contentPadding = PaddingValues(horizontal = 8.dp, vertical = 4.dp),
@@ -126,6 +133,19 @@ fun BillingScreen(
       }
     )
   }
+
+  if (state.pdfData != null) {
+    AlertDialog(
+      onDismissRequest = { viewModel.clearPdf() },
+      title = { Text("Rechnung bereit") },
+      text = { Text("Die Rechnung für ${state.selectedKonto?.personName} wurde generiert (${state.pdfData?.size} Bytes).") },
+      confirmButton = {
+        TextButton(onClick = { viewModel.clearPdf() }) {
+          Text("Schließen")
+        }
+      }
+    )
+  }
 }
 
 @Composable

frontend/features/billing-feature/src/commonMain/kotlin/at/mocode/frontend/features/billing/presentation/BillingViewModel.kt

@@ -16,6 +16,7 @@ data class BillingUiState(
   val konten: List<TeilnehmerKontoDto> = emptyList(),
   val selectedKonto: TeilnehmerKontoDto? = null,
   val buchungen: List<BuchungDto> = emptyList(),
+  val pdfData: ByteArray? = null,
   val error: String? = null
 )
 
@@ -103,7 +104,28 @@ class BillingViewModel(
 
   // Für Abwärtskompatibilität oder Listenansicht (optional)
   fun selectKonto(konto: TeilnehmerKontoDto) {
-    _uiState.value = _uiState.value.copy(selectedKonto = konto)
+    _uiState.value = _uiState.value.copy(selectedKonto = konto, pdfData = null)
     loadBuchungen(konto.id)
   }
+
+  fun downloadRechnung() {
+    val konto = _uiState.value.selectedKonto ?: return
+    viewModelScope.launch {
+      _uiState.value = _uiState.value.copy(isLoading = true, error = null)
+      repository.getRechnungPdf(konto.id)
+        .onSuccess { data ->
+          _uiState.value = _uiState.value.copy(pdfData = data, isLoading = false)
+        }
+        .onFailure {
+          _uiState.value = _uiState.value.copy(
+            isLoading = false,
+            error = "Fehler beim Laden der Rechnung: ${it.message}"
+          )
+        }
+    }
+  }
+
+  fun clearPdf() {
+    _uiState.value = _uiState.value.copy(pdfData = null)
+  }
 }