feat: show success toast after diagram download completes

- Add optional successMessage parameter to saveDiagramToFile()
- Show toast after file download is initiated, not when dialog opens
- Add i18n strings for success message (en/ja/zh)

This is the correct implementation for issue #479 - showing feedback
after the actual save completes rather than when the save dialog opens.

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",