fix: timezone-aware CalDAV sync and English as i18n fallback (#43)

- Apple CalDAV: ICS events with TZID parameter are now converted to UTC
  using the Intl API instead of being stored as floating local time,
  fixing wrong start times for events synced from iOS Calendar
- i18n: fallback language for unsupported browser locales changed from
  German to English

public/i18n.js

@@ -13,7 +13,7 @@ let currentLocale = DEFAULT_LOCALE;
 let translations = {};
 let fallbackTranslations = {};
 
-/** Resolve locale: manual override > navigator.language > default */
+/** Resolve locale: manual override > navigator.language > English > default */
 function resolveLocale() {
   const stored = localStorage.getItem(STORAGE_KEY);
   if (stored && SUPPORTED_LOCALES.includes(stored)) return stored;
@@ -23,7 +23,7 @@ function resolveLocale() {
     const base = tag.split('-')[0].toLowerCase();
     if (SUPPORTED_LOCALES.includes(base)) return base;
   }
-  return DEFAULT_LOCALE;
+  return 'en';
 }
 
 /** Lade eine Locale-JSON-Datei */