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/settings/billing-history.tsx

@@ -0,0 +1,48 @@
+'use client';
+
+import { ExternalLink, Receipt } from 'lucide-react';
+import { useState } from 'react';
+import { Loader2 } from 'lucide-react';
+import { useLanguage } from '@/lib/i18n';
+
+export function BillingHistory() {
+  const { t } = useLanguage();
+  const [loading, setLoading] = useState(false);
+
+  const handleOpenPortal = async () => {
+    setLoading(true);
+    try {
+      const res = await fetch('/api/billing/portal', { method: 'POST' });
+      const data = await res.json();
+      if (!res.ok) throw new Error(data.error ?? 'Failed');
+      window.location.href = data.url;
+    } catch {
+      // ignore — portal not configured
+    } finally {
+      setLoading(false);
+    }
+  };
+
+  return (
+    <div className="rounded-xl border border-border/40 bg-paper p-6 space-y-4">
+      <div className="flex items-center gap-2">
+        <Receipt className="h-4 w-4 text-muted-foreground" />
+        <h3 className="text-sm font-semibold text-foreground">{t('billing.billingHistory')}</h3>
+      </div>
+      <p className="text-xs text-muted-foreground">{t('billing.noUsage')}</p>
+      <button
+        type="button"
+        onClick={handleOpenPortal}
+        disabled={loading}
+        className="flex items-center gap-2 text-xs text-muted-foreground hover:text-foreground transition-colors disabled:opacity-60"
+      >
+        {loading ? (
+          <Loader2 className="h-3.5 w-3.5 animate-spin" />
+        ) : (
+          <ExternalLink className="h-3.5 w-3.5" />
+        )}
+        {t('billing.viewInvoices')}
+      </button>
+    </div>
+  );
+}