fix: markdown-to-richtext conversion failure prevents type switch

- If /api/ai/convert-markdown fails, abort the type change and show error toast
- Only switch to richtext after successful content conversion
- Switching back to markdown always works (no conversion needed)
- Add i18n keys for conversion success/failure messages

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

memento-note/locales/ja.json

@@ -207,6 +207,8 @@
     "typeMarkdown": "Markdown",
     "typeRichText": "リッチテキスト",
     "typeChecklist": "チェックリスト",
+    "convertedToRichText": "Converted to rich text",
+    "conversionFailed": "Conversion failed, staying in Markdown",
     "richTextPlaceholder": "メモを取る...",
     "switchTypeTitle": "ノートタイプを切り替えますか？",
     "switchTypeWarning": "{type} に切り替えると書式が失われる場合があります。",