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/app/vocabulary/components/UnitBuilder.tsx

@@ -43,16 +43,19 @@ interface Props {
   isCreating: boolean
   noSearchResults?: boolean
   searchQuery?: string
+  defaultSourceLang?: string
+  defaultTargetLang?: string
 }
 
 export default function UnitBuilder({
   isDark, glassCard, glassInput,
   selectedWords, onWordsChange, onCreateUnit, isCreating,
   noSearchResults, searchQuery,
+  defaultSourceLang = 'en', defaultTargetLang = 'de',
 }: Props) {
   const [unitTitle, setUnitTitle] = useState('')
-  const [sourceLang, setSourceLang] = useState('de')
-  const [targetLang, setTargetLang] = useState('en')
+  const [sourceLang, setSourceLang] = useState(defaultSourceLang)
+  const [targetLang, setTargetLang] = useState(defaultTargetLang)
   const [showManualEntry, setShowManualEntry] = useState(false)
   const [manualSource, setManualSource] = useState('')
   const [manualTarget, setManualTarget] = useState('')