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/app/onboarding/page.tsx

@@ -15,7 +15,7 @@ interface LangOption {
   rtl: boolean
 }
 
-const STORAGE_KEY = 'bp_native_language'
+const STORAGE_KEY = 'bp_language'
 
 export default function OnboardingPage() {
   const router = useRouter()