Add persistent language switcher across all learn/parent pages

useNativeLanguage: Now has setNativeLang() that persists to localStorage.
Language selection carries across all pages automatically.

LanguageSwitcher: Compact dropdown component added to learn/layout.tsx
and parent/layout.tsx — visible on every sub-page (top-right).

Parent portal: Language dropdown syncs both UI language and native
language. Parents can switch language mid-session (e.g. when both
parents speak different languages).

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

studio-v2/lib/useNativeLanguage.ts

@@ -1,20 +1,26 @@
 'use client'
 
-import { useState, useEffect } from 'react'
+import { useState, useEffect, useCallback } from 'react'
 import { exerciseT, type ExerciseKey } from './exerciseTranslations'
 
 const STORAGE_KEY = 'bp_native_language'
 
 /**
- * Hook to read the user's native language.
- * Returns translation helper for exercise UI texts.
+ * 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, setNativeLang] = useState('de')
+  const [nativeLang, setNativeLangState] = useState('de')
 
   useEffect(() => {
     const stored = localStorage.getItem(STORAGE_KEY)
-    if (stored) setNativeLang(stored)
+    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'
@@ -34,5 +40,5 @@ export function useNativeLanguage() {
     return typeof entry === 'string' ? entry : entry.text || ''
   }
 
-  return { nativeLang, isThirdLanguage, t, wordInNative }
+  return { nativeLang, setNativeLang, isThirdLanguage, t, wordInNative }
 }