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/(main)/layout.tsx

@@ -3,7 +3,8 @@ import { HeaderWrapper } from "@/components/header-wrapper";
 import { Sidebar } from "@/components/sidebar";
 import { ProvidersWrapper } from "@/components/providers-wrapper";
 import { auth } from "@/auth";
-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";
 import { getAISettings } from "@/app/actions/ai-settings";
 
@@ -14,10 +15,14 @@ export default async function MainLayout({
 }: Readonly<{
   children: React.ReactNode;
 }>) {
+  // Read browser language hint from Accept-Language header
+  const headersList = await headers();
+  const browserLang = parseAcceptLanguage(headersList.get("accept-language"));
+
   // Run auth + language detection + translation loading in parallel
   const [session, initialLanguage] = await Promise.all([
     auth(),
-    detectUserLanguage(),
+    detectUserLanguage(browserLang),
   ]);
 
   // Load initial translations server-side to prevent hydration mismatch