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/brainstorm-collab.ts

@@ -1,5 +1,41 @@
 import prisma from '@/lib/prisma'
 
+export type BillingOwnerContext = {
+  billingOwnerId: string
+  isGuestActor: boolean
+}
+
+/** Resolve quota billing owner from an already-loaded session row (no extra query). */
+export function billingOwnerFromSession(
+  sessionUserId: string,
+  requestingUserId: string,
+): BillingOwnerContext {
+  if (!sessionUserId) {
+    throw new Error('Session has no owner — cannot resolve billing owner')
+  }
+  return {
+    billingOwnerId: sessionUserId,
+    isGuestActor: sessionUserId !== requestingUserId,
+  }
+}
+
+/** Host-pays: all collaborative AI usage is billed to BrainstormSession.userId (Story 3.4). */
+export async function getBillingOwner(
+  sessionId: string,
+  requestingUserId: string,
+): Promise<BillingOwnerContext> {
+  const session = await prisma.brainstormSession.findUnique({
+    where: { id: sessionId },
+    select: { userId: true },
+  })
+
+  if (!session) {
+    throw new Error('Session not found')
+  }
+
+  return billingOwnerFromSession(session.userId, requestingUserId)
+}
+
 export async function verifyParticipant(
   sessionId: string,
   userId: string,