- 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>