feat: auto-detect user language from browser settings

Fixes issue where interface always defaulted to English for new users.
Now automatically detects and applies browser language on first visit.

Changes:
- lib/i18n/LanguageProvider.tsx:
  - Add browser language detection using navigator.language
  - Check if detected language is in supported languages list
  - Auto-save detected language to localStorage
  - Update HTML lang attribute for proper font rendering

Behavior:
- First visit: Detects browser language (e.g., fr-FR → fr)
- Returning visits: Uses saved language from localStorage
- Fallback: Defaults to English if language not supported

Result:
✓ French users see French interface automatically
✓ All supported languages work (en, fr, es, de, fa, it, pt, ru, zh, ja, ko, ar, hi, nl, pl)
✓ Better UX for international users

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

keep-notes/lib/i18n/LanguageProvider.tsx

@@ -35,6 +35,16 @@ export function LanguageProvider({ children, initialLanguage = 'en' }: {
     if (saved) {
       setLanguageState(saved)
       document.documentElement.lang = saved
+    } else {
+      // Auto-detect from browser language
+      const browserLang = navigator.language.split('-')[0] as SupportedLanguage
+      const supportedLangs: SupportedLanguage[] = ['en', 'fr', 'es', 'de', 'fa', 'it', 'pt', 'ru', 'zh', 'ja', 'ko', 'ar', 'hi', 'nl', 'pl']
+
+      if (supportedLangs.includes(browserLang)) {
+        setLanguageState(browserLang)
+        localStorage.setItem('user-language', browserLang)
+        document.documentElement.lang = browserLang
+      }
     }
   }, [])