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)/settings/ai/ai-settings-header.tsx

@@ -6,8 +6,8 @@ export function AISettingsHeader() {
   const { t } = useLanguage()
 
   return (
-    <p className="text-[11px] font-bold uppercase tracking-[0.2em] text-muted-foreground">
+    <h3 className="text-[10px] font-bold uppercase tracking-[0.4em] text-concrete opacity-60">
       {t('aiSettings.description')}
-    </p>
+    </h3>
   )
 }