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/ko.json

@@ -207,6 +207,8 @@
     "typeMarkdown": "Markdown",
     "typeRichText": "리치 텍스트",
     "typeChecklist": "체크리스트",
+    "convertedToRichText": "Converted to rich text",
+    "conversionFailed": "Conversion failed, staying in Markdown",
     "richTextPlaceholder": "노트를 작성하세요...",
     "switchTypeTitle": "노트 유형을 변경하시겠습니까?",
     "switchTypeWarning": "{type}(으)로 전환하면 일부 서식이 손실될 수 있습니다.",