Accélérer la capture : viewfinder warmup en arrière-plan

- Supprime sleep(0.2) avant capture et viewfinder off inutile
- Déplace activer_viewfinder + chauffer_liveview dans un thread
  daemon après le save, pour retourner la photo immédiatement
- Supprime pause(150ms) frontend entre fin countdown et capture

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/index.html

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+  <script src="/js/camera.js?v=16"></script>
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