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/components/note-inline-editor.tsx

@@ -497,11 +497,8 @@ export function NoteInlineEditor({
             value={noteType}
             onChange={async (newType) => {
               const oldType = noteType
-              setNoteType(newType)
-              if (newType === 'markdown') setShowMarkdownPreview(true)
-              else setShowMarkdownPreview(false)
 
-              // When switching from markdown to richtext, convert content to HTML
+              // markdown → richtext: convert content to HTML first
               if (oldType === 'markdown' && newType === 'richtext') {
                 try {
                   const res = await fetch('/api/ai/convert-markdown', {
@@ -511,6 +508,8 @@ export function NoteInlineEditor({
                   })
                   if (res.ok) {
                     const { html } = await res.json()
+                    setNoteType('richtext')
+                    setShowMarkdownPreview(false)
                     setContent(html)
                     saveInline(note.id, {
                       type: 'richtext',
@@ -518,13 +517,19 @@ export function NoteInlineEditor({
                       content: html,
                     }).catch(() => {})
                     onChange?.(note.id, { type: 'richtext', isMarkdown: false, content: html })
+                    toast.success(t('notes.convertedToRichText') || 'Converted to rich text')
                     return
                   }
-                } catch {
-                  // Fall through to save without conversion
-                }
+                } catch {}
+                // Conversion failed — abort the type change
+                toast.error(t('notes.conversionFailed') || 'Conversion failed, staying in Markdown')
+                return
               }
 
+              setNoteType(newType)
+              if (newType === 'markdown') setShowMarkdownPreview(true)
+              else setShowMarkdownPreview(false)
+
               // Persist both type and isMarkdown immediately
               saveInline(note.id, {
                 type: newType,