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/app/(main)/settings/billing/page.tsx

@@ -0,0 +1,23 @@
+import { Suspense } from 'react';
+import { Loader2 } from 'lucide-react';
+import { BillingPlans } from '@/components/settings/billing-plans';
+
+export const metadata = {
+  title: 'Billing',
+};
+
+function Fallback() {
+  return (
+    <div className="flex items-center justify-center py-16">
+      <Loader2 className="h-6 w-6 animate-spin text-muted-foreground" />
+    </div>
+  );
+}
+
+export default function BillingPage() {
+  return (
+    <Suspense fallback={<Fallback />}>
+      <BillingPlans />
+    </Suspense>
+  );
+}