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/components/quota-paywall.tsx

@@ -0,0 +1,49 @@
+'use client';
+
+import { Zap } from 'lucide-react';
+import Link from 'next/link';
+import { useLanguage } from '@/lib/i18n';
+
+interface QuotaPaywallProps {
+  feature?: string;
+  onDismiss?: () => void;
+}
+
+export function QuotaPaywall({ feature: _feature, onDismiss }: QuotaPaywallProps) {
+  const { t } = useLanguage();
+
+  return (
+    <div className="rounded-xl border border-[#D4A373]/40 bg-[#D4A373]/5 p-5 space-y-3">
+      <div className="flex items-start gap-3">
+        <div className="mt-0.5 flex h-7 w-7 shrink-0 items-center justify-center rounded-full bg-[#D4A373]/20">
+          <Zap className="h-4 w-4 text-[#D4A373]" />
+        </div>
+        <div className="space-y-1">
+          <p className="text-sm font-semibold text-foreground">
+            {t('billing.upgradeToPro')}
+          </p>
+          <p className="text-xs text-muted-foreground">
+            {t('billing.proDescription')}
+          </p>
+        </div>
+      </div>
+      <div className="flex items-center gap-3">
+        <Link
+          href="/settings/billing"
+          className="px-4 py-1.5 text-xs font-medium rounded-lg bg-[#D4A373] text-white hover:bg-[#C49363] transition-colors"
+        >
+          {t('billing.startCheckout')}
+        </Link>
+        {onDismiss && (
+          <button
+            type="button"
+            onClick={onDismiss}
+            className="text-xs text-muted-foreground hover:text-foreground transition-colors"
+          >
+            {t('common.later') ?? 'Later'}
+          </button>
+        )}
+      </div>
+    </div>
+  );
+}