perf: skip ffmpeg transcode for PocketTTS streaming — use WAV directly

PocketTTS emits 16-bit PCM WAV (16 kHz mono). WAV is natively supported
on all browsers including iOS/macOS Safari, so the ffmpeg MP3 transcode
is unnecessary for the streaming path.

Using format=wav for PocketTTS voices eliminates the ffmpeg subprocess
startup delay (~200–400 ms) and a pipeline stage, giving lower latency
to first audio frame. Kokoro and CF AI continue using MP3 (they output
MP3 natively or via the OpenAI-compatible endpoint).

The runner (MinIO storage) is unaffected — it still stores MP3 for
space efficiency.

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

ui/messages/fr.json

@@ -302,6 +302,24 @@
   "book_detail_range_queuing": "En file d'attente…",
   "book_detail_scrape_range": "Plage d'extraction",
   "book_detail_admin": "Admin",
+  "book_detail_admin_book_cover": "Couverture du livre",
+  "book_detail_admin_chapter_cover": "Couverture du chapitre",
+  "book_detail_admin_chapter_n": "Chapitre n°",
+  "book_detail_admin_description": "Description",
+  "book_detail_admin_chapter_names": "Noms des chapitres",
+  "book_detail_admin_audio_tts": "Audio TTS",
+  "book_detail_admin_voice": "Voix",
+  "book_detail_admin_generate": "Générer",
+  "book_detail_admin_save_cover": "Enregistrer la couverture",
+  "book_detail_admin_saving": "Enregistrement…",
+  "book_detail_admin_saved": "Enregistré",
+  "book_detail_admin_apply": "Appliquer",
+  "book_detail_admin_applying": "Application…",
+  "book_detail_admin_applied": "Appliqué",
+  "book_detail_admin_discard": "Ignorer",
+  "book_detail_admin_enqueue_audio": "Mettre en file audio",
+  "book_detail_admin_cancel_audio": "Annuler",
+  "book_detail_admin_enqueued": "{enqueued} en file, {skipped} ignorés",
   "book_detail_scraping_progress": "Récupération des 20 premiers chapitres. Cette page sera actualisée automatiquement.",
   "book_detail_scraping_home": "← Accueil",
   "book_detail_rescrape_book": "Réextraire le livre",