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/api/billing/portal/route.ts

@@ -0,0 +1,32 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { auth } from '@/auth';
+import { stripe } from '@/lib/stripe';
+import { prisma } from '@/lib/prisma';
+
+export async function POST(req: NextRequest) {
+  const session = await auth();
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  const userId = session.user.id;
+
+  try {
+    const subscription = await prisma.subscription.findUnique({ where: { userId } });
+    if (!subscription?.stripeCustomerId) {
+      return NextResponse.json({ error: 'No active subscription found' }, { status: 404 });
+    }
+
+    const origin = req.headers.get('origin') ?? process.env.NEXTAUTH_URL ?? 'http://localhost:3000';
+
+    const portalSession = await stripe.billingPortal.sessions.create({
+      customer: subscription.stripeCustomerId,
+      return_url: `${origin}/settings/billing`,
+    });
+
+    return NextResponse.json({ url: portalSession.url });
+  } catch (error) {
+    console.error('[billing/portal]', error);
+    return NextResponse.json({ error: 'Failed to create portal session' }, { status: 500 });
+  }
+}