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>

server/src/engine/retro-moves.ts

@@ -122,7 +122,7 @@ export function generateRetroChallenge(): Challenge {
 
   const moveList = knownMoves.map(m => `  ${m.input} = ${m.name} (${m.damage} dmg)`).join('\n')
 
-  const prompt = `RETRO MODE — ARCADE FIGHT!\n\nEnter 3 gamepad combos separated by |\nButtons: ↑ ↓ ← → A B\n\nKNOWN MOVES:\n${moveList}\n\nSECRET COMBOS exist! Longer button chains = more damage. Experiment!\n\nFormat: combo1 | combo2 | combo3\nExample: ↓→+A | B | →→+A`
+  const prompt = `ARCADE ROUND! Pick 3 moves:\n${moveList}\n\nSecret combos exist — longer chains hit harder.\nAnswer: combo1 | combo2 | combo3`
 
   return {
     type: 'retro_mode',