feat(backups): Show how much space a service uses on backup

- Resolve: #434

lib/ui/pages/backups/create_backups_modal.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 import 'package:selfprivacy/logic/bloc/backups/backups_bloc.dart';
 import 'package:selfprivacy/logic/bloc/server_jobs/server_jobs_bloc.dart';
+import 'package:selfprivacy/logic/bloc/volumes/volumes_bloc.dart';
 import 'package:selfprivacy/logic/models/json/server_job.dart';
 import 'package:selfprivacy/logic/models/service.dart';
 
@@ -103,6 +104,23 @@ class _CreateBackupsModalState extends State<CreateBackupsModal> {
         ...widget.services.map(
           (final Service service) {
             final bool busy = busyServices.contains(service.id);
+            String description;
+            if (busy) {
+              description = 'backup.service_busy'.tr();
+            } else {
+              description = service.backupDescription;
+              description += '\n';
+              description += 'service_page.uses'.tr(
+                namedArgs: {
+                  'usage': service.storageUsage.used.toString(),
+                  'volume': context
+                      .read<VolumesBloc>()
+                      .state
+                      .getVolume(service.storageUsage.volume ?? '')
+                      .displayName,
+                },
+              );
+            }
             return CheckboxListTile.adaptive(
               onChanged: !busy
                   ? (final bool? value) {
@@ -123,7 +141,7 @@ class _CreateBackupsModalState extends State<CreateBackupsModal> {
                 service.displayName,
               ),
               subtitle: Text(
-                busy ? 'backup.service_busy'.tr() : service.backupDescription,
+                description,
               ),
               secondary: SvgPicture.string(
                 service.svgIcon,