Fix: Language switch takes effect immediately (React Context)

Replaced localStorage-only hook with React Context Provider.
Layout and page components now share the same state — switching
language in the dropdown instantly updates all text on screen
without requiring a page reload.

NativeLanguageProvider added to root layout.tsx.
useNativeLanguage() re-exported from Context for backward compat.

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

studio-v2/lib/useNativeLanguage.ts

@@ -1,44 +1,4 @@
 'use client'
 
-import { useState, useEffect, useCallback } from 'react'
-import { exerciseT, type ExerciseKey } from './exerciseTranslations'
-
-const STORAGE_KEY = 'bp_native_language'
-
-/**
- * Hook for native language state + translations.
- * Persists to localStorage. Can be changed at any time (e.g. parent switches language).
- */
-export function useNativeLanguage() {
-  const [nativeLang, setNativeLangState] = useState('de')
-
-  useEffect(() => {
-    const stored = localStorage.getItem(STORAGE_KEY)
-    if (stored) setNativeLangState(stored)
-  }, [])
-
-  /** Change native language (persists to localStorage) */
-  const setNativeLang = useCallback((lang: string) => {
-    setNativeLangState(lang)
-    localStorage.setItem(STORAGE_KEY, lang)
-  }, [])
-
-  const isThirdLanguage = nativeLang !== 'de' && nativeLang !== 'en'
-
-  /** Get translated exercise UI text */
-  const t = (key: ExerciseKey): string => {
-    const entry = exerciseT[key]
-    if (!entry) return key
-    return (entry as Record<string, string>)[nativeLang] || entry.de || key
-  }
-
-  /** Get native translation of a vocab word from translations JSONB */
-  const wordInNative = (translations?: Record<string, any>): string => {
-    if (!translations || !isThirdLanguage) return ''
-    const entry = translations[nativeLang]
-    if (!entry) return ''
-    return typeof entry === 'string' ? entry : entry.text || ''
-  }
-
-  return { nativeLang, setNativeLang, isThirdLanguage, t, wordInNative }
-}
+// Re-export from Context so all existing imports keep working
+export { useNativeLanguage } from './NativeLanguageContext'