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/lib/i18n/detect-user-language.ts

@@ -1,6 +1,9 @@
 /**
- * Detect user's preferred language from their existing notes
- * Uses a single DB-level GROUP BY query — no note content is loaded
+ * Detect user's preferred language.
+ * Priority:
+ * 1. Most common language among user's notes (DB GROUP BY)
+ * 2. Browser language hint (passed from server component via Accept-Language)
+ * 3. Default: 'en'
  */
 
 import { auth } from '@/auth'
@@ -10,10 +13,37 @@ import { SupportedLanguage } from './load-translations'
 
 const SUPPORTED_LANGUAGES = new Set(['en', 'fr', 'es', 'de', 'fa', 'it', 'pt', 'ru', 'zh', 'ja', 'ko', 'ar', 'hi', 'nl', 'pl'])
 
+/**
+ * Parse an Accept-Language header string and find the best matching supported language
+ */
+export function parseAcceptLanguage(acceptLanguage: string | null): SupportedLanguage | null {
+  if (!acceptLanguage) return null
+
+  // Parse Accept-Language: "fr-FR,fr;q=0.9,en-US;q=0.8,en;q=0.7"
+  const languages = acceptLanguage
+    .split(',')
+    .map(lang => {
+      const [code, q] = lang.trim().split(';q=')
+      return { code: code.trim().toLowerCase(), quality: q ? parseFloat(q) : 1.0 }
+    })
+    .sort((a, b) => b.quality - a.quality)
+
+  for (const { code } of languages) {
+    if (SUPPORTED_LANGUAGES.has(code)) {
+      return code as SupportedLanguage
+    }
+    const base = code.split('-')[0]
+    if (SUPPORTED_LANGUAGES.has(base)) {
+      return base as SupportedLanguage
+    }
+  }
+
+  return null
+}
+
 const getCachedUserLanguage = unstable_cache(
-  async (userId: string): Promise<SupportedLanguage> => {
+  async (userId: string): Promise<SupportedLanguage | null> => {
     try {
-      // Single aggregated query — no notes are fetched, only language counts
       const result = await prisma.note.groupBy({
         by: ['language'],
         where: {
@@ -33,22 +63,39 @@ const getCachedUserLanguage = unstable_cache(
         }
       }
 
-      return 'en'
+      return null
     } catch (error) {
-      console.error('Error detecting user language:', error)
-      return 'en'
+      console.error('Error detecting user language from notes:', error)
+      return null
     }
   },
   ['user-language'],
   { tags: ['user-language'] }
 )
 
-export async function detectUserLanguage(): Promise<SupportedLanguage> {
+/**
+ * Detect user language.
+ * @param browserLanguageHint - Optional browser language parsed from Accept-Language header.
+ *                               Should be passed from server components that have access to headers().
+ */
+export async function detectUserLanguage(browserLanguageHint?: SupportedLanguage | null): Promise<SupportedLanguage> {
   const session = await auth()
 
   if (!session?.user?.id) {
-    return 'en'
+    return browserLanguageHint || 'en'
   }
 
-  return getCachedUserLanguage(session.user.id)
+  // 1. Try to detect from user's notes
+  const noteLanguage = await getCachedUserLanguage(session.user.id)
+  if (noteLanguage) {
+    return noteLanguage
+  }
+
+  // 2. Fall back to browser language hint
+  if (browserLanguageHint) {
+    return browserLanguageHint
+  }
+
+  // 3. Default
+  return 'en'
 }