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/note-editor/note-editor-toolbar.tsx

@@ -165,7 +165,7 @@ export function NoteEditorToolbar({ mode, onClose, onToggleAttachments, attachme
               if (!seed.trim()) return
               window.open(`/brainstorm?seed=${encodeURIComponent(seed.slice(0, 300))}&sourceNoteId=${note.id}`, '_self')
             }}
-            className="p-1.5 rounded-full border border-orange-300 dark:border-orange-700 text-orange-500 hover:bg-orange-50 dark:hover:bg-orange-900/20 transition-all"
+            className="p-1.5 rounded-full border border-brand-accent/30 dark:border-brand-accent/50 text-brand-accent hover:bg-brand-accent/10 dark:hover:bg-brand-accent/20 transition-all"
           >
             <Wind size={16} />
           </button>