Unify language system: one setting for all modules

- Merge two separate language systems (bp_language + bp_native_language) into one
- NativeLanguageContext now reads from LanguageContext (same localStorage key)
- Extend i18n.ts to 26 languages with flags (UI falls back to EN/DE)
- Replace LanguageSwitcher with LanguageDropdown (flags) in learn + parent layouts
- Migration: old bp_native_language value auto-migrates to bp_language
- Onboarding page writes to bp_language (unified key)

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

studio-v2/lib/LanguageContext.tsx

@@ -43,9 +43,9 @@ export function LanguageProvider({ children }: { children: ReactNode }) {
     }
   }
 
-  // Uebersetzungsfunktion
+  // Uebersetzungsfunktion (Fallback: en → de → key)
   const t = (key: string): string => {
-    return translations[language][key] || translations[defaultLanguage][key] || key
+    return translations[language]?.[key] || translations['en']?.[key] || translations['de']?.[key] || key
   }
 
   // Waehrend SSR: Default anzeigen
@@ -55,7 +55,7 @@ export function LanguageProvider({ children }: { children: ReactNode }) {
         value={{
           language: defaultLanguage,
           setLanguage: () => {},
-          t: (key) => translations[defaultLanguage][key] || key,
+          t: (key) => translations[defaultLanguage]?.[key] || key,
           isRTL: false,
           availableLanguages,
         }}