Fix TTS stop on navigation and answer; set Polly Joanna as default

- Add key prop to TTSButton based on question id + answer state
  so component remounts (and audio stops) on both question navigation
  and answer selection in study mode
- Set AWS Polly Joanna (neural) as default TTS voice

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

frontend/src/pages/QuizPage.jsx

@@ -286,6 +286,7 @@ useEffect(() => {
           <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'flex-start' }}>
             <h3 style={{ flex: 1 }}>Q{currentIdx + 1}. {current.question_text.replace('[IMAGE]', '')}</h3>
             <TTSButton
+              key={`${current.id}_${answers[current.id] || ''}`}
               text={`Question ${currentIdx + 1}. ${current.question_text.replace('[IMAGE]', '')}. Options: ${(current.options || []).join(', ')}`}
               voice={selectedVoice}
               onActiveChange={setTtsActive}