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/(main)/layout.tsx

@@ -7,6 +7,7 @@ import { detectUserLanguage, parseAcceptLanguage } from "@/lib/i18n/detect-user-
 import { loadTranslations } from "@/lib/i18n/load-translations";
 import { getAISettings } from "@/app/actions/ai-settings";
 import { AIChatLayoutBridge } from "@/components/ai-chat-layout-bridge";
+import { PageTransition } from "@/components/page-transition";
 
 export default async function MainLayout({
   children,
@@ -37,7 +38,9 @@ export default async function MainLayout({
         </Suspense>
 
         <main className="flex min-h-0 flex-1 flex-col overflow-y-auto scroll-smooth bg-memento-paper dark:bg-background">
-          {children}
+          <PageTransition>
+            {children}
+          </PageTransition>
         </main>
 
         {showAIAssistant && <AIChatLayoutBridge />}