feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

memento-note/lib/i18n/load-translations.ts

@@ -919,6 +919,53 @@ export interface Translations {
     expand: string
     collapse: string
   }
+  billing: {
+    title: string
+    currentPlan: string
+    upgradePlan: string
+    manageBilling: string
+    manageDescription: string
+    openPortal: string
+    renewsOn: string
+    expiresOn: string
+    canceledAt: string
+    freePlan: string
+    proPlan: string
+    businessPlan: string
+    enterprisePlan: string
+    perMonth: string
+    perYear: string
+    monthly: string
+    annual: string
+    save: string
+    upgradeTitle: string
+    proPrice: string
+    businessPrice: string
+    proAnnualPrice: string
+    businessAnnualPrice: string
+    proFeature1: string
+    proFeature2: string
+    proFeature3: string
+    proFeature4: string
+    businessFeature1: string
+    businessFeature2: string
+    businessFeature3: string
+    businessFeature4: string
+    enterpriseTitle: string
+    enterpriseDescription: string
+    contactSales: string
+    startCheckout: string
+    checkoutLoading: string
+    checkoutSuccess: string
+    checkoutCanceled: string
+    active: string
+    trialing: string
+    pastDue: string
+    canceled: string
+    inactive: string
+    billingEnabled: string
+    billingDisabled: string
+  }
   [key: string]: any
 }