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/components/rich-text-editor.tsx

@@ -41,6 +41,7 @@ import { parseBlockReferenceFromText, recallLastBlockReference, type ParsedBlock
 import { getEmptyParagraphAtSelection } from '@/lib/editor/empty-paragraph-at-selection'
 import { SmartPasteExtension } from '@/lib/editor/smart-paste-extension'
 import { BlockSelectionExtension } from '@/lib/editor/block-selection-extension'
+import { MarkdownPasteExtension } from '@/lib/editor/markdown-paste-extension'
 import { TurnIntoShortcutExtension } from '@/lib/editor/turn-into-shortcut-extension'
 import { UndoRedoFeedbackExtension } from '@/lib/editor/undo-redo-feedback-extension'
 import type { Node as PMNode } from '@tiptap/pm/model'
@@ -417,6 +418,7 @@ export const RichTextEditor = forwardRef<RichTextEditorHandle, RichTextEditorPro
         ...globalDragHandleExtensions,
         SmartPasteExtension,
         BlockSelectionExtension,
+        MarkdownPasteExtension,
         TurnIntoShortcutExtension,
         UndoRedoFeedbackExtension,
         LiveBlockExtension,