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/label-management-dialog.tsx

@@ -135,13 +135,13 @@ export function LabelManagementDialog(props: LabelManagementDialogProps = {}) {
                   <div key={label.id} className="flex items-center justify-between p-2 rounded-md hover:bg-accent/50 group">
                     <div className="flex items-center gap-3 flex-1 relative">
                       {isAI ? (
-                        <Sparkles className={cn("h-4 w-4", "text-memento-blue")} />
+                        <Sparkles className={cn("h-4 w-4", "text-brand-accent")} />
                       ) : (
                         <div className={cn("h-3 w-3 rounded-full", colorClasses.bg)} />
                       )}
                       <span className="font-medium text-sm">{label.name}</span>
                       {isAI && (
-                        <span className="text-[8px] px-1.5 py-0.5 rounded-full bg-memento-blue/10 text-memento-blue font-bold uppercase">IA</span>
+                        <span className="text-[8px] px-1.5 py-0.5 rounded-full bg-brand-accent/10 text-brand-accent font-bold uppercase">IA</span>
                       )}
 
                       {isEditing && (