Show success toast after saving diagram (#484)

* Add success toast after saving diagram

* fix: correct save toast placement

* Changes made:
1. Added i18n support
2. Fixed the issue where the save toast was running only once

* fix: show toast after download completes, not when dialog opens

Move toast from handleDrawioSave (dialog open) to saveDiagramToFile
(after download). Also restore the duplicate-save guard that was removed.

---------

Co-authored-by: Biki Kalita <86558912+Biki-dev@users.noreply.github.com>
Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

lib/i18n/dictionaries/ja.json

@@ -117,7 +117,8 @@
             "drawio": "Draw.io XML",
             "png": "PNG 画像",
             "svg": "SVG 画像"
-        }
+        },
+        "savedSuccessfully": "保存完了！"
     },
     "history": {
         "title": "ダイアグラム履歴",