feat(12-01): update i18n store lang init to use navigator.language fallback

- Replace hardcoded 'fr' default with IIFE that checks localStorage first
- Falls back to navigator.language: 'en' if starts with 'en', else 'fr'
- localStorage preference always wins over browser language setting

imptune/templates/base.html

@@ -25,7 +25,11 @@
 
     // i18n store: FR/EN toggle, persists in localStorage
     Alpine.store('i18n', {
-      lang: localStorage.getItem('imptune_lang') || 'fr',
+      lang: (() => {
+        const saved = localStorage.getItem('imptune_lang');
+        if (saved) return saved;
+        return navigator.language && navigator.language.startsWith('en') ? 'en' : 'fr';
+      })(),
       t(key) {
         return (this.translations[this.lang] || {})[key] || key;
       },