feat: 8 AI providers, rich text editor, agent notifications, UI contrast & font settings

- Add DeepSeek, OpenRouter, Mistral, Z.AI, LM Studio as AI providers
  with editable model names via Combobox in admin settings
- Fix OpenRouter broken by normalizeProvider bug in config.ts
- Convert agent-created notes from Markdown to HTML (TipTap rich text)
- Add Notification model + in-app notifications for agent results
- Agent notification click opens the created note directly
- Add note count display on notebook and inbox headers
- Fix checklist toggle in card view (persist state via localCheckItems)
- Add checklist creation option in tabs/list view (dropdown on + button)
- Fix image description ENOENT error with HTTP fallback
- Improve UI contrast across all themes (input, border, checkbox visibility)
- Add font family setting (Inter vs System Default) in Appearance settings
- Fix CSS font-sans variable conflict (removed dead Geist references)
- Update README with new features and 8 providers

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

memento-note/app/(admin)/layout.tsx

@@ -1,5 +1,6 @@
 import { AdminProvidersWrapper } from '@/components/admin-providers-wrapper'
-import { detectUserLanguage } from '@/lib/i18n/detect-user-language'
+import { headers } from 'next/headers'
+import { detectUserLanguage, parseAcceptLanguage } from '@/lib/i18n/detect-user-language'
 import { loadTranslations } from '@/lib/i18n/load-translations'
 
 // No <Suspense> here intentionally: combining a Suspense boundary with <Link>
@@ -10,7 +11,9 @@ export default async function AdminGroupLayout({
 }: {
   children: React.ReactNode
 }) {
-  const initialLanguage = await detectUserLanguage()
+  const headersList = await headers()
+  const browserLang = parseAcceptLanguage(headersList.get('accept-language'))
+  const initialLanguage = await detectUserLanguage(browserLang)
   const initialTranslations = await loadTranslations(initialLanguage)
 
   return (