fix(item-form): make kcal label dynamic based on selected unit

Analogous to the price label, kcal now shows 'kcal pro [Einheit]'
based on the dropdown selection from #114.

Closes #115

app/src/main/java/de/bollwerk/app/ui/item/ItemFormScreen.kt

@@ -199,7 +199,14 @@ internal fun ItemFormScreen(
             OutlinedTextField(
                 value = uiState.kcalPerUnit,
                 onValueChange = viewModel::updateKcalPerUnit,
-                label = { Text("kcal / Einheit") },
+                label = {
+                    val unitLabel = if (uiState.isCustomUnit) {
+                        uiState.customUnit.ifBlank { "Einheit" }
+                    } else {
+                        uiState.unit
+                    }
+                    Text("kcal pro $unitLabel")
+                },
                 keyboardOptions = KeyboardOptions(keyboardType = KeyboardType.Number),
                 singleLine = true,
                 modifier = Modifier.fillMaxWidth()