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-manager.tsx

@@ -201,7 +201,7 @@ export function LabelManager({ existingLabels, notebookId, onUpdate }: LabelMana
                         className={cn(
                           'inline-flex items-center gap-1.5 rounded-full border px-2.5 py-0.5 text-[9px] font-bold uppercase tracking-wider cursor-pointer',
                           isAI
-                            ? 'bg-memento-blue/5 border-memento-blue/20 text-memento-blue'
+                            ? 'bg-brand-accent/5 border-brand-accent/20 text-brand-accent'
                             : `${colorClasses.bg} ${colorClasses.text} ${colorClasses.border}`
                         )}
                         onClick={() => setEditingColor(isEditing ? null : label)}
@@ -244,7 +244,7 @@ export function LabelManager({ existingLabels, notebookId, onUpdate }: LabelMana
                         className={cn(
                           'inline-flex items-center gap-1.5 rounded-full border px-2.5 py-0.5 text-[9px] font-bold uppercase tracking-wider cursor-pointer transition-all hover:opacity-80',
                           isAI
-                            ? 'bg-memento-blue/5 border-memento-blue/20 text-memento-blue'
+                            ? 'bg-brand-accent/5 border-brand-accent/20 text-brand-accent'
                             : `${colorClasses.bg} ${colorClasses.text} ${colorClasses.border}`
                         )}
                         onClick={() => handleSelectExisting(label.name)}