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/lib/theme-script.ts

@@ -27,6 +27,8 @@ export function getThemeScript(serverTheme: string = 'light') {
       root.setAttribute('data-theme', theme);
       if (theme === 'midnight') root.classList.add('dark');
     }
+    var accentStored = localStorage.getItem('accent-color');
+    if (accentStored) root.style.setProperty('--color-brand-accent', accentStored);
   } catch (e) {
     console.error('Theme script error', e);
   }