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/parent/layout.tsx

@@ -2,16 +2,14 @@
 
 import { Sidebar } from '@/components/Sidebar'
 import { useTheme } from '@/lib/ThemeContext'
-import { useNativeLanguage } from '@/lib/useNativeLanguage'
-import { LanguageSwitcher } from '@/components/learn/LanguageSwitcher'
+import { LanguageDropdown } from '@/components/LanguageDropdown'
 
 /**
  * Shared layout for ALL /parent/* pages.
- * Same design as learn layout — Sidebar + gradient + language switcher.
+ * Same design as learn layout — Sidebar + gradient + flag language dropdown.
  */
 export default function ParentLayout({ children }: { children: React.ReactNode }) {
   const { isDark } = useTheme()
-  const { nativeLang, setNativeLang } = useNativeLanguage()
 
   return (
     <div className={`min-h-screen flex relative overflow-hidden ${
@@ -23,13 +21,8 @@ export default function ParentLayout({ children }: { children: React.ReactNode }
         <Sidebar />
       </div>
       <div className="flex-1 flex flex-col relative z-10 overflow-y-auto scrollbar-hide" style={{ scrollbarWidth: 'none' }}>
-        {/* Sticky language switcher at top-right */}
         <div className="sticky top-0 z-20 flex justify-end px-4 py-2">
-          <LanguageSwitcher
-            currentLang={nativeLang}
-            onLangChange={setNativeLang}
-            isDark={isDark}
-          />
+          <LanguageDropdown />
         </div>
         {children}
       </div>