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/components/brainstorm/activity-feed.tsx

@@ -14,7 +14,7 @@ interface ActivityFeedProps {
 
 function getActionIcon(action: string) {
   switch (action) {
-    case 'manual_idea': return <Lightbulb size={12} className="text-memento-blue" />
+    case 'manual_idea': return <Lightbulb size={12} className="text-brand-accent" />
     case 'wave_generated': return <Zap size={12} className="text-orange-500" />
     case 'joined': return <UserPlus size={12} className="text-emerald-500" />
     case 'idea_dismissed': return <X size={12} className="text-rose-500" />