fix(sonar): remove unused byId map in cast spans (javascript:S4030)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

static/js/audiobook-cast-spans.js

@@ -54,8 +54,6 @@ async function audiobookCastSpans(overrideUrl, overrideModel, resume) {
   const spans = S.segmentByQuotes(text);
   if (!spans || !spans.some(s => s.kind === 'dialogue')) return false;   // no quote style: classic engine
   const windows = S.buildWindows(spans, text);
-  const byId = new Map();
-  for (const w of windows) for (const sp of w.spans) byId.set(sp.id, sp);
 
   _audiobook.running = true;
   _audiobook.castMode = 'spans';