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-lock.json

@@ -66,10 +66,12 @@
         "@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",
@@ -118,6 +120,8 @@
         "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",
@@ -128,6 +132,7 @@
         "@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",
@@ -2728,6 +2733,12 @@
         "langium": "3.3.1"
       }
     },
+    "node_modules/@mixmark-io/domino": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/@mixmark-io/domino/-/domino-2.2.0.tgz",
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+      "license": "BSD-2-Clause"
+    },
     "node_modules/@mozilla/readability": {
       "version": "0.6.0",
       "resolved": "https://registry.npmjs.org/@mozilla/readability/-/readability-0.6.0.tgz",
@@ -8270,6 +8281,13 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/canvas-confetti": {
+      "version": "1.9.0",
+      "resolved": "https://registry.npmjs.org/@types/canvas-confetti/-/canvas-confetti-1.9.0.tgz",
+      "integrity": "sha512-aBGj/dULrimR1XDZLtG9JwxX1b4HPRF6CX9Yfwh3NvstZEm1ZL7RBnel4keCPSqs1ANRu1u2Aoz9R+VmtjYuTg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/chai": {
       "version": "5.2.3",
       "resolved": "https://registry.npmjs.org/@types/chai/-/chai-5.2.3.tgz",
@@ -8738,6 +8756,12 @@
       "license": "MIT",
       "optional": true
     },
+    "node_modules/@types/turndown": {
+      "version": "5.0.6",
+      "resolved": "https://registry.npmjs.org/@types/turndown/-/turndown-5.0.6.tgz",
+      "integrity": "sha512-ru00MoyeeouE5BX4gRL+6m/BsDfbRayOskWqUvh7CLGW+UXxHQItqALa38kKnOiZPqJrtzJUgAC2+F0rL1S4Pg==",
+      "license": "MIT"
+    },
     "node_modules/@types/unist": {
       "version": "3.0.3",
       "resolved": "https://registry.npmjs.org/@types/unist/-/unist-3.0.3.tgz",
@@ -10165,6 +10189,16 @@
         "node": ">=12"
       }
     },
+    "node_modules/canvas-confetti": {
+      "version": "1.9.4",
+      "resolved": "https://registry.npmjs.org/canvas-confetti/-/canvas-confetti-1.9.4.tgz",
+      "integrity": "sha512-yxQbJkAVrFXWNbTUjPqjF7G+g6pDotOUHGbkZq2NELZUMDpiJ85rIEazVb8GTaAptNW2miJAXbs1BtioA251Pw==",
+      "license": "ISC",
+      "funding": {
+        "type": "donate",
+        "url": "https://www.paypal.me/kirilvatev"
+      }
+    },
     "node_modules/canvas-roundrect-polyfill": {
       "version": "0.0.1",
       "resolved": "https://registry.npmjs.org/canvas-roundrect-polyfill/-/canvas-roundrect-polyfill-0.0.1.tgz",
@@ -19065,6 +19099,25 @@
         "zustand": "^4.3.2"
       }
     },
+    "node_modules/turndown": {
+      "version": "7.2.4",
+      "resolved": "https://registry.npmjs.org/turndown/-/turndown-7.2.4.tgz",
+      "integrity": "sha512-I8yFsfRzmzK0WV1pNNOA4A7y4RDfFxPRxb3t+e3ui14qSGOxGtiSP6GjeX+Y6CHb7HYaFj7ECUD7VE5kQMZWGQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@mixmark-io/domino": "^2.2.0"
+      },
+      "engines": {
+        "node": ">=18",
+        "npm": ">=9"
+      }
+    },
+    "node_modules/turndown-plugin-gfm": {
+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/turndown-plugin-gfm/-/turndown-plugin-gfm-1.0.2.tgz",
+      "integrity": "sha512-vwz9tfvF7XN/jE0dGoBei3FXWuvll78ohzCZQuOb+ZjWrs3a0XhQVomJEb2Qh4VHTPNRO4GPZh0V7VRbiWwkRg==",
+      "license": "MIT"
+    },
     "node_modules/tw-animate-css": {
       "version": "1.4.0",
       "resolved": "https://registry.npmjs.org/tw-animate-css/-/tw-animate-css-1.4.0.tgz",