fix(chart): transaction mapping fix + add cache for suggestions

- Fix transaction.mapping.map usage - it returns a position, not mapping
- Add 5-minute in-memory cache for chart suggestions (max 50 entries)
- Cache key based on content hash + selection
- Significantly improves UX when re-analyzing same content

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

memento-note/components/rich-text-editor.tsx

@@ -297,7 +297,7 @@ export const RichTextEditor = forwardRef<RichTextEditorHandle, RichTextEditorPro
       try {
         console.log('[handleSelectChart] Inserting chart type:', chartContent.split('\n')[0])
 
-        const { from, to } = editor.state.selection
+        const { from } = editor.state.selection
         const { tr } = editor.state
         const { schema } = editor.state
 
@@ -313,17 +313,21 @@ export const RichTextEditor = forwardRef<RichTextEditorHandle, RichTextEditorPro
           return
         }
 
-        // Replace selection with the chart node
-        const transaction = tr.replaceWith(from, to, chartNode)
+        // Insert the chart node at current position
+        const transaction = tr.insert(from, chartNode)
+
+        // Get the position after the inserted chart
+        const insertPos = transaction.mapping.map(from)
+        const chartSize = chartNode.nodeSize
 
         // Add a paragraph after for continued typing
         const paragraph = schema.nodes.paragraph.create()
-        transaction.insert(transaction.mapping.map(to).map(to), paragraph)
+        transaction.insert(insertPos + chartSize, paragraph)
 
         editor.view.dispatch(transaction)
         editor.chain().focus().run()
 
-        console.log('[handleSelectChart] Chart inserted')
+        console.log('[handleSelectChart] Chart inserted successfully')
       } catch (error) {
         console.error('[handleSelectChart] Failed:', error)
         toast.error('Failed to insert chart: ' + (error as Error).message)