fix: kill in-flight SFX when navigating away from fight

Disconnect and recreate sfxGain in stopAllAudio() so oscillators
and noise buffers stop immediately instead of playing to completion.

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

frontend/src/game/sounds.ts

@@ -340,6 +340,14 @@ export function stopAllAudio() {
   stopMusic()
   if (typeof speechSynthesis !== 'undefined') speechSynthesis.cancel()
   _speechQueueDepth = 0
+  // Disconnect sfxGain to instantly kill all in-flight oscillators/buffers,
+  // then reconnect so future sounds still work
+  if (sfxGain && ctx) {
+    sfxGain.disconnect()
+    sfxGain = ctx.createGain()
+    sfxGain.gain.value = masterMuted ? 0 : 0.25
+    sfxGain.connect(ctx.destination)
+  }
 }
 
 // Public voice functions