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/app/api/brainstorm/[sessionId]/export-pptx/route.ts

@@ -0,0 +1,65 @@
+import { NextRequest, NextResponse } from 'next/server'
+import prisma from '@/lib/prisma'
+import { auth } from '@/auth'
+import { generateBrainstormPptx } from '@/lib/brainstorm/export-pptx'
+
+export async function POST(
+  _request: NextRequest,
+  { params }: { params: Promise<{ sessionId: string }> }
+) {
+  const session = await auth()
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  try {
+    const { sessionId } = await params
+
+    const brainstormSession = await prisma.brainstormSession.findFirst({
+      where: {
+        id: sessionId,
+        OR: [
+          { userId: session.user.id },
+          { participants: { some: { userId: session.user.id } } },
+        ],
+      },
+      include: {
+        ideas: {
+          orderBy: [{ waveNumber: 'asc' }, { createdAt: 'asc' }],
+        },
+      },
+    })
+
+    if (!brainstormSession) {
+      return NextResponse.json({ error: 'Session not found' }, { status: 404 })
+    }
+
+    const sessionData = {
+      seedIdea: brainstormSession.seedIdea,
+      createdAt: brainstormSession.createdAt,
+      ideas: brainstormSession.ideas.map(idea => ({
+        id: idea.id,
+        title: idea.title,
+        description: idea.description,
+        waveNumber: idea.waveNumber,
+        status: idea.status,
+        isStarred: idea.isStarred,
+        convertedToNoteId: idea.convertedToNoteId,
+      })),
+    }
+
+    const { buffer, filename } = await generateBrainstormPptx(sessionData)
+
+    return new NextResponse(new Uint8Array(buffer), {
+      status: 200,
+      headers: {
+        'Content-Type': 'application/vnd.openxmlformats-officedocument.presentationml.presentation',
+        'Content-Disposition': `attachment; filename="${filename}"`,
+        'Content-Length': String(buffer.length),
+      },
+    })
+  } catch (err) {
+    console.error('[export-pptx]', err)
+    return NextResponse.json({ error: 'Export failed' }, { status: 500 })
+  }
+}