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/package.json

@@ -87,10 +87,12 @@
     "@tiptap/y-tiptap": "^3.0.3",
     "@types/d3": "^7.4.3",
     "@types/jsdom": "^28.0.1",
+    "@types/turndown": "^5.0.6",
     "ai": "^6.0.23",
     "autoprefixer": "^10.4.23",
     "bcryptjs": "^3.0.3",
     "buffer": "^6.0.3",
+    "canvas-confetti": "^1.9.4",
     "cheerio": "^1.1.2",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
@@ -139,6 +141,8 @@
     "tinyld": "^1.3.4",
     "tiptap-extension-auto-joiner": "^0.1.3",
     "tiptap-extension-global-drag-handle": "^0.1.18",
+    "turndown": "^7.2.4",
+    "turndown-plugin-gfm": "^1.0.2",
     "vazirmatn": "^33.0.3",
     "y-protocols": "^1.0.7",
     "yjs": "^13.6.30",
@@ -149,6 +153,7 @@
     "@tailwindcss/postcss": "^4",
     "@tailwindcss/typography": "^0.5.19",
     "@types/bcryptjs": "^2.4.6",
+    "@types/canvas-confetti": "^1.9.0",
     "@types/dagre": "^0.7.54",
     "@types/diff": "^7.0.2",
     "@types/ioredis": "^4.28.10",