feat(ai): add AI chart suggestions in TipTap editor

Implement slash command "/suggest-charts" that analyzes note content
and suggests 3 appropriate chart types with visual previews.

Created:
- chart-suggestion.tool.ts: AI tool for data extraction and chart recommendations
- suggest-charts/route.ts: API endpoint for chart suggestions
- chart-suggestion.service.ts: Frontend service layer
- chart-suggestions-dialog.tsx: Modal with 3 chart proposals and thumbnails
- tiptap-chart-extension.tsx: TipTap Node extension for rendering chart blocks

Modified:
- rich-text-editor.tsx: Added slash command, toolbar button, and dialog integration

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

memento-note/lib/ai/services/chart-suggestion.service.ts

@@ -0,0 +1,80 @@
+/**
+ * Chart Suggestion Service
+ * Frontend service for calling the AI chart suggestions API
+ */
+
+export interface ChartSuggestion {
+  type: 'bar' | 'horizontal-bar' | 'line' | 'area' | 'pie' | 'radar' | 'funnel' | 'gauge'
+  title: string
+  data: { label: string; value: number }[]
+  description: string
+  rationale?: string
+}
+
+export interface SuggestChartsResponse {
+  suggestions: ChartSuggestion[]
+  analyzedText: string
+  detectedData: string
+  hasData: boolean
+  error?: string
+}
+
+export interface SuggestChartsRequest {
+  content: string
+  selection?: string | null
+  noteId?: string
+}
+
+/**
+ * Call the AI chart suggestions API
+ * @param request - The request parameters
+ * @returns Promise with the chart suggestions
+ */
+export async function suggestCharts(request: SuggestChartsRequest): Promise<SuggestChartsResponse> {
+  try {
+    const response = await fetch('/api/ai/suggest-charts', {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+      },
+      body: JSON.stringify(request),
+    })
+
+    if (!response.ok) {
+      const errorData = await response.json().catch(() => ({ error: 'Unknown error' }))
+      return {
+        suggestions: [],
+        analyzedText: '',
+        detectedData: '',
+        hasData: false,
+        error: errorData.error || `HTTP ${response.status}`,
+      }
+    }
+
+    const data = await response.json()
+    return data as SuggestChartsResponse
+  } catch (error) {
+    console.error('[suggestCharts] Network error:', error)
+    return {
+      suggestions: [],
+      analyzedText: '',
+      detectedData: '',
+      hasData: false,
+      error: error instanceof Error ? error.message : 'Network error',
+    }
+  }
+}
+
+/**
+ * Convert a chart suggestion to markdown code block format
+ * @param suggestion - The chart suggestion to convert
+ * @returns Markdown code block string
+ */
+export function chartSuggestionToMarkdown(suggestion: ChartSuggestion): string {
+  const lines = [
+    suggestion.type,
+    suggestion.title,
+    ...suggestion.data.map(d => `${d.label}: ${d.value}`),
+  ]
+  return `\`\`\`chart\n${lines.join('\n')}\n\`\`\``
+}