fix: TTS sentence cutoff + simplified retro challenge prompt

TTS fixes:
- Remove 60-char truncation in FightViewer — let speakAnswer handle limits
- Smart truncation at sentence boundaries (period, comma, etc.) up to 200 chars
- Chrome keepalive: periodic pause/resume prevents silent 15s cutoff bug
- Deduplicated cleanup logic in speakAsync/speakAsyncWithRate

Retro mode:
- Simplified challenge prompt from verbose wall of text to clean 3-line format

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

frontend/src/components/FightViewer.vue

@@ -322,7 +322,7 @@ async function _doReplay() {
         scene.showSpeechBubble('a', round.botAResponse.slice(0, 60), 5)
         scene.startTalking('a')
       }
-      if (doTTS) await speakAnswer(props.fight.botA!.name, round.botAResponse.slice(0, 60))
+      if (doTTS) await speakAnswer(props.fight.botA!.name, round.botAResponse)
       else await sleep(insanityMode.value ? 30 : 800)
       scene?.stopTalking('a')
       if (!insanityMode.value) await sleep(150)
@@ -337,7 +337,7 @@ async function _doReplay() {
         scene.showSpeechBubble('b', round.botBResponse.slice(0, 60), 5)
         scene.startTalking('b')
       }
-      if (doTTS) await speakAnswer(props.fight.botB!.name, round.botBResponse.slice(0, 60))
+      if (doTTS) await speakAnswer(props.fight.botB!.name, round.botBResponse)
       else await sleep(insanityMode.value ? 30 : 800)
       scene?.stopTalking('b')
       if (!insanityMode.value) await sleep(150)