Epic 6: Stories 6-2 (Markdown roundtrip) + 6-3 (Brainstorm PPTX + Canvas)

Story 6-2 — Markdown roundtrip export/import:
- lib/editor/markdown-export.ts: tiptapHTMLToMarkdown, markdownToHTML, looksLikeMarkdown
- lib/editor/markdown-paste-extension.ts: TipTap extension paste Markdown → blocs
- note-editor-toolbar.tsx: export .md + import .md (file picker)
- rich-text-editor.tsx: intégration MarkdownPasteExtension
- 40 tests unitaires markdown-export.test.ts

Story 6-3 — Brainstorm PPTX + Canvas:
- lib/brainstorm/export-pptx.ts: génération PPTX 5 slides (pptxgenjs)
- app/api/brainstorm/[sessionId]/export-pptx/route.ts: route POST protégée
- brainstorm-page.tsx: bouton PPTX, auto-select session, fix emoji, fix router.replace
- wave-canvas.tsx: fitTrigger recentrage, légende bas-droite

Onboarding activation wizard (Story 6-1):
- components/onboarding/: wizard multi-étapes, hints éditeur
- app/api/onboarding/: route PATCH onboarding
- prisma/migrations: champs onboarding user

Locales: 15 langues mises à jour (brainstorm, markdown, onboarding keys)
Sprint: 6-1 done, 6-2 review, 6-3 review

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

memento-note/lib/editor/markdown-export.ts

@@ -0,0 +1,209 @@
+/**
+ * markdown-export.ts
+ * Utilities for TipTap HTML ↔ Markdown conversion.
+ *
+ * Uses:
+ *   - turndown (+ turndown-plugin-gfm) : HTML → Markdown
+ *   - marked                           : Markdown → HTML
+ */
+
+import TurndownService from 'turndown'
+import { tables, taskListItems, strikethrough } from 'turndown-plugin-gfm'
+import { marked } from 'marked'
+
+// ── Markdown heuristic detection ────────────────────────────────────────────
+
+const MARKDOWN_PATTERNS = [
+  /^#{1,6}\s/m,           // headings
+  /^\s*[-*+]\s/m,         // unordered list
+  /^\s*\d+\.\s/m,         // ordered list
+  /^\s*>\s/m,             // blockquote
+  /^```/m,                // code fence
+  /`[^`]+`/,              // inline code
+  /\*\*[^*]+\*\*/,        // bold
+  /\*[^*]+\*/,            // italic
+  /^[|].+[|]/m,           // table
+  /\[.+\]\(.+\)/,         // link
+  /!\[.+\]\(.+\)/,        // image
+  /~~[^~]+~~/,            // strikethrough
+]
+
+/**
+ * Returns true if the given plain text looks like it contains Markdown syntax.
+ * Used by the paste handler to decide whether to convert before inserting.
+ */
+export function looksLikeMarkdown(text: string): boolean {
+  if (!text || text.trim().length < 3) return false
+  return MARKDOWN_PATTERNS.some((re) => re.test(text))
+}
+
+// ── Turndown service factory ─────────────────────────────────────────────────
+
+function createTurndownService(): TurndownService {
+  const td = new TurndownService({
+    headingStyle: 'atx',
+    hr: '---',
+    bulletListMarker: '-',
+    codeBlockStyle: 'fenced',
+    fence: '```',
+    emDelimiter: '_',
+    strongDelimiter: '**',
+    linkStyle: 'inlined',
+    linkReferenceStyle: 'full',
+  })
+
+  // GFM plugins: tables + task lists + strikethrough
+  td.use([tables, taskListItems, strikethrough])
+
+  // Custom rule: liveBlock → HTML comment
+  td.addRule('liveBlock', {
+    filter(node) {
+      return (
+        node.nodeName === 'DIV' &&
+        (node as HTMLElement).hasAttribute('data-live-block')
+      )
+    },
+    replacement(_content, node) {
+      const el = node as HTMLElement
+      const sourceNoteId = el.getAttribute('sourcenoteId') || el.getAttribute('sourcenoteId') || el.getAttribute('sourcenoteid') || ''
+      const blockId = el.getAttribute('blockId') || el.getAttribute('blockid') || ''
+      return `\n\n<!-- live-block: ${sourceNoteId}#${blockId} -->\n\n`
+    },
+  })
+
+  // Custom rule: structuredViewBlock → HTML comment
+  td.addRule('structuredViewBlock', {
+    filter(node) {
+      return (
+        node.nodeName === 'DIV' &&
+        (node as HTMLElement).hasAttribute('data-structured-view-block')
+      )
+    },
+    replacement(_content, node) {
+      const el = node as HTMLElement
+      const attrs: Record<string, string> = {}
+      for (const attr of Array.from(el.attributes)) {
+        if (attr.name !== 'data-structured-view-block') {
+          attrs[attr.name] = attr.value
+        }
+      }
+      return `\n\n<!-- structured-view: ${JSON.stringify(attrs)} -->\n\n`
+    },
+  })
+
+  return td
+}
+
+// Singleton (lazy-init) — safe for server + client usage
+let _tdService: TurndownService | null = null
+function getTurndownService(): TurndownService {
+  if (!_tdService) _tdService = createTurndownService()
+  return _tdService
+}
+
+// ── Custom node pre-processor ─────────────────────────────────────────────
+
+// Sentinel prefix — alphanumeric only to avoid Markdown escaping by turndown
+const SENTINEL_PREFIX = 'MOMENTOBLOCKSENTINEL'
+
+interface BlockPlaceholder {
+  key: string
+  comment: string
+}
+
+/**
+ * Pre-process HTML before passing to turndown:
+ * - Replace empty custom node divs (liveBlock, structuredViewBlock) with text
+ *   placeholders so they survive turndown processing (turndown drops blank nodes
+ *   and strips HTML comments).
+ * - Return the modified HTML and a map of placeholder → HTML comment.
+ */
+function preprocessCustomNodes(html: string): { html: string; placeholders: BlockPlaceholder[] } {
+  const placeholders: BlockPlaceholder[] = []
+
+  // liveBlock: <div data-live-block="true" sourceNoteId="..." blockId="..."></div>
+  let result = html.replace(
+    /<div([^>]*?data-live-block[^>]*?)>\s*<\/div>/gi,
+    (_match, attrs) => {
+      const snId = (attrs.match(/sourcenoteid="([^"]*)"/i) || attrs.match(/sourcenoteid='([^']*)'/i) || [])[1] || ''
+      const bId = (attrs.match(/blockid="([^"]*)"/i) || attrs.match(/blockid='([^']*)'/i) || [])[1] || ''
+      const key = `${SENTINEL_PREFIX}LIVEBLOCK${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- live-block: ${snId}#${bId} -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  // structuredViewBlock: <div data-structured-view-block="true" ...></div>
+  result = result.replace(
+    /<div([^>]*?data-structured-view-block[^>]*?)>\s*<\/div>/gi,
+    (_match, attrs) => {
+      const attrMap: Record<string, string> = {}
+      const attrRe = /(data-[a-z-]+)="([^"]*)"/gi
+      let m: RegExpExecArray | null
+      while ((m = attrRe.exec(attrs)) !== null) {
+        if (m[1] !== 'data-structured-view-block') attrMap[m[1]] = m[2]
+      }
+      const key = `${SENTINEL_PREFIX}SVBLOCK${placeholders.length}`
+      placeholders.push({ key, comment: `<!-- structured-view: ${JSON.stringify(attrMap)} -->` })
+      return `<p>${key}</p>`
+    }
+  )
+
+  return { html: result, placeholders }
+}
+
+/**
+ * Post-process the markdown output: replace sentinel placeholders with HTML comments.
+ */
+function postprocessPlaceholders(md: string, placeholders: BlockPlaceholder[]): string {
+  let result = md
+  for (const { key, comment } of placeholders) {
+    result = result.replace(key, `\n\n${comment}\n\n`)
+  }
+  return result
+}
+
+// ── HTML → Markdown ──────────────────────────────────────────────────────────
+
+/**
+ * Convert a TipTap-generated HTML string to GitHub-Flavored Markdown.
+ * Custom nodes (liveBlock, structuredViewBlock) are serialised as HTML comments.
+ */
+export function tiptapHTMLToMarkdown(html: string): string {
+  if (!html || html.trim() === '') return ''
+  const { html: preprocessed, placeholders } = preprocessCustomNodes(html)
+  const td = getTurndownService()
+  const md = td.turndown(preprocessed).trim()
+  return postprocessPlaceholders(md, placeholders).trim()
+}
+
+// ── Markdown → HTML ──────────────────────────────────────────────────────────
+
+/**
+ * Convert a Markdown string to HTML suitable for injection into TipTap via
+ * `editor.commands.setContent(html)`.
+ *
+ * Uses marked with GFM enabled (tables, task lists, line breaks).
+ */
+export function markdownToHTML(markdown: string): string {
+  if (!markdown || markdown.trim() === '') return ''
+
+  // marked v18+ uses synchronous parse by default when no async tokens
+  const html = marked.parse(markdown, {
+    gfm: true,
+    breaks: false,
+  }) as string
+
+  return html
+}
+
+// ── Title extraction from Markdown ──────────────────────────────────────────
+
+/**
+ * Extract the first H1 title from a Markdown string.
+ * Returns null if no H1 is found.
+ */
+export function extractMarkdownTitle(markdown: string): string | null {
+  const match = markdown.match(/^#\s+(.+)/m)
+  return match ? match[1].trim() : null
+}