feat(story-3.6): complete Stripe subscription tiers — enterprise card, build fix, i18n

Story 3.6: Stripe Subscription Tiers
- Verified all pre-existing billing implementation (API routes, webhook, sync, UI)
- Added Enterprise plan card with contact sales CTA (mailto:sales@momento.app)
- Fixed lib/stripe.ts build error (lazy getStripe() + placeholder default)
- Added enterpriseFeature1-5 i18n keys to all 15 locales
- 22/22 unit tests pass, build succeeds
- Story status: ready-for-dev → review

memento-note/components/settings/billing-plans.tsx

@@ -204,6 +204,26 @@ export function BillingPlans() {
         : 'bg-ink text-white shadow-xl shadow-ink/20 hover:scale-[1.02] active:scale-95',
       onClick: () => handleCheckout('BUSINESS'),
     },
+    {
+      id: 'enterprise',
+      name: t('billing.enterpriseTitle') || 'Enterprise',
+      price: t('billing.contactSales') || 'Sur devis',
+      period: '',
+      description: t('billing.enterpriseDescription') || 'Quotas personnalisés, SSO, support prioritaire.',
+      features: [
+        t('billing.enterpriseFeature1') || 'Quotas illimités',
+        t('billing.enterpriseFeature2') || 'SSO / SAML',
+        t('billing.enterpriseFeature3') || 'Support dédié',
+        t('billing.enterpriseFeature4') || 'Facturation personnalisée',
+        t('billing.enterpriseFeature5') || 'SLA garanti',
+      ],
+      current: effectiveTier === 'ENTERPRISE',
+      buttonText: effectiveTier === 'ENTERPRISE' ? (t('billing.currentPlan') || 'Plan Actuel') : (t('billing.contactSales') || 'Contact Sales'),
+      buttonClass: effectiveTier === 'ENTERPRISE'
+        ? 'bg-paper text-concrete cursor-default'
+        : 'bg-ink text-white shadow-xl shadow-ink/20 hover:scale-[1.02] active:scale-95',
+      onClick: () => { window.location.href = 'mailto:sales@momento.app'; },
+    },
   ];
 
   return (
@@ -362,7 +382,7 @@ export function BillingPlans() {
 
       {/* Plan Cards */}
       {(billingEnabled || true) && (
-        <div className="grid grid-cols-1 lg:grid-cols-3 gap-8">
+        <div className="grid grid-cols-1 lg:grid-cols-4 gap-6">
           {plans.map((plan) => (
             <div
               key={plan.id}