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/notification-panel.tsx

@@ -200,7 +200,7 @@ export function NotificationPanel() {
     <Popover open={open} onOpenChange={setOpen}>
       <PopoverTrigger asChild>
         <button
-          className="relative h-9 w-9 flex items-center justify-center rounded-full bg-white border border-border text-muted-foreground hover:text-foreground hover:bg-white/40 transition-all"
+          className="relative p-1.5 text-muted-ink hover:text-ink transition-all hover:bg-white/50 dark:hover:bg-white/10 rounded-lg border border-transparent hover:border-border"
         >
           <Bell className="h-4 w-4 transition-transform duration-200 hover:scale-110" />
           {pendingCount > 0 && (