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/en.json

@@ -302,6 +302,24 @@
   "book_detail_range_queuing": "Queuing\u2026",
   "book_detail_scrape_range": "Scrape range",
   "book_detail_admin": "Admin",
+  "book_detail_admin_book_cover": "Book Cover",
+  "book_detail_admin_chapter_cover": "Chapter Cover",
+  "book_detail_admin_chapter_n": "Chapter #",
+  "book_detail_admin_description": "Description",
+  "book_detail_admin_chapter_names": "Chapter Names",
+  "book_detail_admin_audio_tts": "Audio TTS",
+  "book_detail_admin_voice": "Voice",
+  "book_detail_admin_generate": "Generate",
+  "book_detail_admin_save_cover": "Save Cover",
+  "book_detail_admin_saving": "Saving…",
+  "book_detail_admin_saved": "Saved",
+  "book_detail_admin_apply": "Apply",
+  "book_detail_admin_applying": "Applying…",
+  "book_detail_admin_applied": "Applied",
+  "book_detail_admin_discard": "Discard",
+  "book_detail_admin_enqueue_audio": "Enqueue Audio",
+  "book_detail_admin_cancel_audio": "Cancel",
+  "book_detail_admin_enqueued": "Enqueued {enqueued}, skipped {skipped}",
   "book_detail_scraping_progress": "Fetching the first 20 chapters. This page will refresh automatically.",
   "book_detail_scraping_home": "\u2190 Home",
   "book_detail_rescrape_book": "Rescrape book",