Add Schulsprache (school language) as second language setting

- LanguageContext: new schoolLanguage + setSchoolLanguage
- LanguageDropdown: two tabs (Muttersprache / Schulsprache) with flag selection
- UnitBuilder: defaults target language to schoolLanguage
- Stored in bp_school_language localStorage (default: de)
- Shows school flag badge next to main language when different

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/lib/i18n.ts

@@ -388,8 +388,9 @@ export const translations: Record<string, Record<string, string>> = {
   },
 }
 
-// LocalStorage Key
+// LocalStorage Keys
 const STORAGE_KEY = 'bp_language'
+const SCHOOL_LANG_KEY = 'bp_school_language'
 
 // Aktuelle Sprache aus localStorage oder default
 export function getStoredLanguage(): Language {
@@ -413,6 +414,19 @@ export function setStoredLanguage(lang: Language): void {
   localStorage.setItem(STORAGE_KEY, lang)
 }
 
+// Schulsprache aus localStorage (default: 'de')
+export function getStoredSchoolLanguage(): Language {
+  if (typeof window === 'undefined') return defaultLanguage
+  const stored = localStorage.getItem(SCHOOL_LANG_KEY)
+  if (stored && stored in availableLanguages) return stored
+  return defaultLanguage
+}
+
+export function setStoredSchoolLanguage(lang: Language): void {
+  if (typeof window === 'undefined') return
+  localStorage.setItem(SCHOOL_LANG_KEY, lang)
+}
+
 // Uebersetzung abrufen (Fallback: en → de → key)
 export function t(key: string, lang: Language): string {
   return translations[lang]?.[key] || translations['en']?.[key] || translations['de']?.[key] || key