fix: slide 3 noire, watermark PPTX, quota génération slides

- canvas-board.tsx: préfère data.html (iframe) sur data.spec (ancien renderer)
  → corrige slide 3 noire en mode HTML viewer
- pptx/route.ts: ajoute watermark 'memento-note.com' sur chaque slide
  via buildPptx (PPTX téléchargé depuis le canvas)
- run-for-note/route.ts: checkEntitlementOrThrow avant création agent
- slides.tool.ts: incrementUsageAsync après canvas créé avec succès

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

memento-note/app/api/agents/run-for-note/route.ts

@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { auth } from '@/auth'
 import { prisma } from '@/lib/prisma'
+import { checkEntitlementOrThrow, QuotaExceededError, incrementUsageAsync } from '@/lib/entitlements'
 
 type GenerateType = 'slide-generator' | 'excalidraw-generator'
 
@@ -43,6 +44,16 @@ export async function POST(req: NextRequest) {
     return NextResponse.json({ error: 'Paramètres invalides' }, { status: 400 })
   }
 
+  // Quota check — slides/excalidraw generation counts as 'reformulate' credit
+  try {
+    await checkEntitlementOrThrow(userId, 'reformulate')
+  } catch (e) {
+    if (e instanceof QuotaExceededError) {
+      return NextResponse.json({ error: e.message }, { status: 402 })
+    }
+    throw e
+  }
+
   const note = await prisma.note.findFirst({
     where: { id: noteId, userId },
     select: { id: true, title: true, notebookId: true },