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/locales/fr.json

@@ -2361,7 +2361,12 @@
     "billing": "Facturation",
     "renewal": "Renouvellement",
     "paidPlanDesc": "Votre abonnement se renouvelle automatiquement.",
-    "businessDescription": "Pour les équipes et chefs de produit."
+    "businessDescription": "Pour les équipes et chefs de produit.",
+    "enterpriseFeature1": "Quotas illimités",
+    "enterpriseFeature2": "SSO / SAML",
+    "enterpriseFeature3": "Support dédié",
+    "enterpriseFeature4": "Facturation personnalisée",
+    "enterpriseFeature5": "SLA garanti"
   },
   "landing": {
     "nav": {