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/fr.json

@@ -207,6 +207,8 @@
     "typeMarkdown": "Markdown",
     "typeRichText": "Texte riche",
     "typeChecklist": "Liste de tâches",
+    "convertedToRichText": "Converti en texte enrichi",
+    "conversionFailed": "Échec de la conversion, reste en Markdown",
     "richTextPlaceholder": "Prenez une note...",
     "switchTypeTitle": "Changer le type de note ?",
     "switchTypeWarning": "Certaines mises en forme peuvent être perdues lors du passage en {type}.",