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/hooks/use-brainstorm.ts

@@ -2,8 +2,26 @@
 
 import { useQueryClient, useMutation, useQuery } from '@tanstack/react-query'
 import { queryKeys } from '@/lib/query-keys'
+import {
+  BrainstormQuotaError,
+  type BrainstormQuotaPayload,
+} from '@/lib/brainstorm-quota-client'
 import type { BrainstormSession, BrainstormSessionListItem } from '@/types/brainstorm'
 
+function parseBrainstormQuota(res: Response, data: BrainstormQuotaPayload & Record<string, unknown>) {
+  if (res.status === 402 && data?.error === 'QUOTA_EXCEEDED') {
+    throw new BrainstormQuotaError(data, 'QUOTA_EXCEEDED')
+  }
+}
+
+/** i18n key for host-pays quota toasts (Story 3.4). */
+export function brainstormQuotaMessageKey(err: unknown): string | null {
+  if (err instanceof BrainstormQuotaError) {
+    return err.isGuestActor ? 'brainstorm.quotaGuest' : 'brainstorm.quotaHost'
+  }
+  return null
+}
+
 export interface CreateBrainstormResult {
   session: BrainstormSession
   contextSummary?: {
@@ -79,6 +97,7 @@ export function useCreateBrainstorm() {
         body: JSON.stringify(input),
       })
       const data = await res.json()
+      parseBrainstormQuota(res, data)
       if (!data.success) throw new Error(data.error || 'Failed to create brainstorm')
       return { session: data.data, contextSummary: data.contextSummary }
     },
@@ -100,6 +119,7 @@ export function useExpandIdea(sessionId: string) {
         body: JSON.stringify(input),
       })
       const data = await res.json()
+      parseBrainstormQuota(res, data)
       if (!data.success) throw new Error(data.error || 'Failed to expand idea')
       return data.data
     },
@@ -258,6 +278,7 @@ export function useAddManualIdea(sessionId: string) {
         body: JSON.stringify(input),
       })
       const data = await res.json()
+      parseBrainstormQuota(res, data)
       if (!data.success) throw new Error(data.error || 'Failed to add idea')
       return data.data
     },