feat(credits): solde IA unique (option M), packs Stripe et polish billing

Un pot de crédits global (BASIC 100 / PRO 1 000 / BUSINESS 4 000) remplace
les seaux par fonction côté débit. Packs one-shot S/M/L, admin sans seaux
legacy, usage multi-features, hints de coût, anti-flash thème et hydratation
admin. Inclut aussi le pipeline slides renforcé et la page publique polishée.

memento-note/components/block-action-menu.tsx

@@ -406,7 +406,12 @@ export function BlockActionMenu({
       {/* Création de diagramme */}
       <button type="button" className="block-action-item" onClick={() => { void handleCreateDiagram() }}>
         <Sparkles size={16} className="text-amber-500 transition-all duration-200" />
-        <span className="font-medium text-amber-700 dark:text-amber-400">{t('blockAction.createDiagram')}</span>
+        <span className="font-medium text-amber-700 dark:text-amber-400 flex flex-col items-start gap-0.5">
+          <span>{t('blockAction.createDiagram')}</span>
+          <span className="text-[9px] font-normal text-muted-foreground normal-case tracking-normal">
+            {t('blockAction.createDiagramCostHint')}
+          </span>
+        </span>
       </button>
 
       <div className="block-action-separator" />