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-quota-client.ts

@@ -0,0 +1,35 @@
+/** Client-side parsing for brainstorm host-pays quota responses (Story 3.4). */
+
+export type BrainstormQuotaPayload = {
+  error: string
+  feature?: string
+  upgradeTier?: string
+  byokConfigured?: boolean
+  billingOwnerId?: string
+  triggeredByUserId?: string
+  isGuestActor?: boolean
+}
+
+export class BrainstormQuotaError extends Error {
+  readonly code = 'QUOTA_EXCEEDED'
+  readonly isGuestActor: boolean
+  readonly payload: BrainstormQuotaPayload
+
+  constructor(payload: BrainstormQuotaPayload, message: string) {
+    super(message)
+    this.name = 'BrainstormQuotaError'
+    this.isGuestActor = Boolean(payload.isGuestActor)
+    this.payload = payload
+  }
+}
+
+export function throwIfBrainstormQuota(
+  res: Response,
+  data: BrainstormQuotaPayload & { success?: boolean; error?: string },
+  guestMessage: string,
+  hostMessage: string,
+): void {
+  if (res.status !== 402 || data?.error !== 'QUOTA_EXCEEDED') return
+  const message = data.isGuestActor ? guestMessage : hostMessage
+  throw new BrainstormQuotaError(data, message)
+}