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/app/(main)/settings/ai/ai-settings-header.tsx

@@ -6,8 +6,8 @@ export function AISettingsHeader() {
   const { t } = useLanguage()
 
   return (
-    <p className="text-[11px] font-bold uppercase tracking-[0.2em] text-muted-foreground">
+    <h3 className="text-[10px] font-bold uppercase tracking-[0.4em] text-concrete opacity-60">
       {t('aiSettings.description')}
-    </p>
+    </h3>
   )
 }

memento-note/app/(main)/settings/ai/page.tsx

@@ -1,6 +1,7 @@
 import { auth } from '@/auth'
 import { redirect } from 'next/navigation'
 import { AISettingsPanel } from '@/components/ai/ai-settings-panel'
+import { ByokSettingsPanel } from '@/components/ai/byok-settings-panel'
 import { getAISettings } from '@/app/actions/ai-settings'
 import { AISettingsHeader } from './ai-settings-header'
 
@@ -17,6 +18,7 @@ export default async function AISettingsPage() {
     <div className="space-y-6">
       <AISettingsHeader />
       <AISettingsPanel initialSettings={settings} />
+      <ByokSettingsPanel />
     </div>
   )
 }