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/components/lab/canvas-board.tsx

@@ -218,11 +218,10 @@ function SlidesViewer({ data, name, canvasId }: { data: SlidesPayload; name: str
           </button>
         </div>
       </div>
-      {/* Slides: React renderer (legacy spec) or iframe (new HTML) */}
+      {/* Slides: iframe (new HTML format) preferred over legacy React renderer */}
       <div className="flex-1 relative overflow-hidden group">
-        {data.spec ? (
-          <SlidesRenderer spec={data.spec} />
-        ) : (
+        {data.html ? (
+          // New format: standalone animated HTML served via srcDoc
           <>
             {/* Loading overlay — visible until iframe fires onLoad */}
             {!isLoaded && (
@@ -257,7 +256,10 @@ function SlidesViewer({ data, name, canvasId }: { data: SlidesPayload; name: str
               <ChevronRight className="w-5 h-5" />
             </button>
           </>
-        )}
+        ) : data.spec ? (
+          // Legacy format: old React renderer (recharts)
+          <SlidesRenderer spec={data.spec} />
+        ) : null}
       </div>
     </div>
   )