Wire all 7 learn pages to central translation system

All exercise pages now use useNativeLanguage() hook:
- Buttons show text in user's native language (Richtig→Dogru, etc.)
- Instructions translated (Geschafft→Bitti, Nochmal→Tekrar, etc.)
- wordInNative() available for vocab translations (needs data)

Pages updated: flashcards, quiz, type, listen, match, pronounce, story.

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

studio-v2/app/learn/[unitId]/flashcards/page.tsx

@@ -3,6 +3,7 @@
 import React, { useState, useEffect, useCallback } from 'react'
 import { useParams, useRouter } from 'next/navigation'
 import { useTheme } from '@/lib/ThemeContext'
+import { useNativeLanguage } from '@/lib/useNativeLanguage'
 import { FlashCard } from '@/components/learn/FlashCard'
 import { AudioButton } from '@/components/learn/AudioButton'
 
@@ -23,6 +24,7 @@ export default function FlashcardsPage() {
   const { unitId } = useParams<{ unitId: string }>()
   const router = useRouter()
   const { isDark } = useTheme()
+  const { t, wordInNative, nativeLang, isThirdLanguage } = useNativeLanguage()
 
   const [items, setItems] = useState<QAItem[]>([])
   const [currentIndex, setCurrentIndex] = useState(0)