feat: server-side TTS audio generation, audio queue panel, and home screen fix

- Add POST /ui/audio/{slug}/{n} endpoint: calls Kokoro-FastAPI server-side,
  writes MP3 atomically to disk, deduplicates concurrent requests via
  in-flight channel map, wraps route with 10-min TimeoutHandler
- Add GET /ui/audio-file/{slug}/{n} endpoint: serves cached MP3 with 1-day
  cache headers
- Add AudioDir/AudioPath helpers to writer.go
- Rewrite JS TTS: remove all MSE/blob/stream code; use plain fetch to
  generate endpoint then set audio.src to returned URL
- Add AbortController to generateAudio; abort on stop() and navigation
- Keep voice/speed controls disabled until setPlaying() fires
- Reset prefetchFired on voice/speed change
- Upgrade prefetch from fire-and-forget to promise chain updating queue badge
- Add sticky bottom audio queue panel: always-visible scrubber with timestamps,
  expandable rows for Now/Next/Dbg with color-coded state badges
- Fix iOS Reader Mode: header->nav aria-hidden, audio outside nav, role=main,
  visible h1 in content area, hover-only paragraph highlight
- Fix home screen Available books hidden: stop hiding cards in #books-grid
  when they appear in Continue Reading; Available always shows all books

scraper/internal/writer/writer.go

@@ -408,6 +408,26 @@ func (w *Writer) bookDir(slug string) string {
 	return filepath.Join(w.root, slug)
 }
 
+// AudioDir returns the directory used to cache generated MP3 files for a book.
+func (w *Writer) AudioDir(slug string) string {
+	return filepath.Join(w.bookDir(slug), "audio")
+}
+
+// AudioPath returns the full path for a cached chapter audio file.
+// The filename is keyed by chapter number, voice, and speed so that different
+// settings never collide.  Speed is formatted to one decimal place (e.g. "1.0").
+func (w *Writer) AudioPath(slug string, n int, voice string, speed float64) string {
+	// Sanitise voice so it is safe as a filename component.
+	safeVoice := strings.Map(func(r rune) rune {
+		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '_' || r == '-' {
+			return r
+		}
+		return '_'
+	}, voice)
+	filename := fmt.Sprintf("ch%d-%s-%.1f.mp3", n, safeVoice, speed)
+	return filepath.Join(w.AudioDir(slug), filename)
+}
+
 // chapterPath computes the full file path for a chapter.
 //
 //	vol-{volume}/{folderRange}/chapter-{number}.md