Add snackbar notification when download zip completed/failed

lib/dialog/download_zip_page.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/app_localizations.dart';
@@ -103,8 +104,21 @@ class _DownloadZipPage extends State<DownloadZipPage> {
                                   jpnTitle: useTitleJpn,
                                   exportAd: exportAd,
                                   maxLength: int.tryParse(maxLength))
-                              .catchError((err) {
+                              .then((_) {
+                            if (!kIsWeb) {
+                              final snack = SnackBar(
+                                  content: Text(i18n.downloadComplete));
+                              rootScaffoldMessengerKey.currentState
+                                  ?.showSnackBar(snack);
+                            }
+                          }).catchError((err) {
                             _log.warning("Failed to download zip:", err);
+                            if (!kIsWeb) {
+                              final snack = SnackBar(
+                                  content: Text(i18n.downloadZipFailed));
+                              rootScaffoldMessengerKey.currentState
+                                  ?.showSnackBar(snack);
+                            }
                           });
                           context.canPop()
                               ? context.pop()

lib/l10n/app_en.arb

@@ -82,5 +82,7 @@
     },
     "downloadAsZip": "Download as ZIP file",
     "exportAd": "Export pages which marked as ads",
-    "maxZipFilenameLength": "Maximum length of filenames in Zip files"
+    "maxZipFilenameLength": "Maximum length of filenames in Zip files",
+    "downloadComplete": "Download completed.",
+    "downloadZipFailed": "Failed to download ZIP file."
 }

lib/l10n/app_zh.arb

@@ -82,5 +82,7 @@
     },
     "downloadAsZip": "下载为ZIP文件",
     "exportAd": "导出标记为广告的页面",
-    "maxZipFilenameLength": "Zip文件中文件名的最大长度"
+    "maxZipFilenameLength": "Zip文件中文件名的最大长度",
+    "downloadComplete": "下载完毕。",
+    "downloadZipFailed": "Zip文件下载失败。"
 }