feat: N9 audio polish — speech robustness, crossfade, volume balancing

- visibilitychange handler resumes speech + AudioContext on tab return
- speak() retry logic: re-attempts once on non-cancel errors after 200ms
- VOICE_VOLUME_SCALE (0.7) prevents voice from overpowering SFX/music
- Music crossfade: brief gain dip (40% -> 100%) on track switches
- Applied volume scale to all speech paths (speak, announce custom)

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

frontend/src/game/sounds.ts

@@ -280,9 +280,20 @@ if (typeof speechSynthesis !== 'undefined') {
     if (speechSynthesis.speaking && !speechSynthesis.paused) return
     if (speechSynthesis.paused) speechSynthesis.resume()
   }, 5000)
+  // Resume speech + audio when tab becomes visible again
+  if (typeof document !== 'undefined') {
+    document.addEventListener('visibilitychange', () => {
+      if (document.visibilityState === 'visible') {
+        if (speechSynthesis.paused) speechSynthesis.resume()
+        if (ctx?.state === 'suspended') ctx.resume().catch(() => {})
+      }
+    })
+  }
 }
 
 let _speechQueueDepth = 0
+// Scale speech volume down so voice doesn't overpower SFX/music
+const VOICE_VOLUME_SCALE = 0.7
 
 function speak(text: string, profileName: string, cancelPrevious: boolean = false, _echo: boolean = false) {
   if (typeof speechSynthesis === 'undefined') return
@@ -302,10 +313,26 @@ function speak(text: string, profileName: string, cancelPrevious: boolean = fals
   if (profile.voice) utter.voice = profile.voice
   utter.pitch = profile.pitch
   utter.rate = profile.rate
-  utter.volume = profile.volume
+  utter.volume = profile.volume * VOICE_VOLUME_SCALE
   _speechQueueDepth++
   utter.onend = () => { _speechQueueDepth = Math.max(0, _speechQueueDepth - 1) }
-  utter.onerror = () => { _speechQueueDepth = Math.max(0, _speechQueueDepth - 1) }
+  utter.onerror = (ev) => {
+    _speechQueueDepth = Math.max(0, _speechQueueDepth - 1)
+    // Retry once on non-cancel errors (interrupted = browser killed it, not us)
+    if (ev.error !== 'canceled' && ev.error !== 'interrupted') {
+      setTimeout(() => {
+        if (!masterMuted && typeof speechSynthesis !== 'undefined') {
+          const retry = new SpeechSynthesisUtterance(text)
+          if (profile.voice) retry.voice = profile.voice
+          retry.pitch = profile.pitch; retry.rate = profile.rate; retry.volume = profile.volume * VOICE_VOLUME_SCALE
+          _speechQueueDepth++
+          retry.onend = () => { _speechQueueDepth = Math.max(0, _speechQueueDepth - 1) }
+          retry.onerror = () => { _speechQueueDepth = Math.max(0, _speechQueueDepth - 1) }
+          speechSynthesis.speak(retry)
+        }
+      }, 200)
+    }
+  }
   speechSynthesis.speak(utter)
 }
 
@@ -326,7 +353,7 @@ export function announce(text: string, pitch?: number, rate?: number) {
     if (profile.voice) utter.voice = profile.voice
     utter.pitch = pitch ?? 1.0
     utter.rate = rate ?? 0.8
-    utter.volume = 1.0
+    utter.volume = VOICE_VOLUME_SCALE
     _speechQueueDepth++
     utter.onend = () => { _speechQueueDepth = Math.max(0, _speechQueueDepth - 1) }
     utter.onerror = () => { _speechQueueDepth = Math.max(0, _speechQueueDepth - 1) }
@@ -1748,6 +1775,7 @@ function playMusicBar() {
       const others = pool.filter(t => t !== activeTrack)
       return others.length > 0 ? others[Math.floor(Math.random() * others.length)] : pool[0]
     }
+    const prevTrack = activeTrack
     if (currentIntensity > 0.7) {
       activeTrack = pickFrom(intenseTracks.length > 0 ? intenseTracks : ALL_TRACKS)
     } else if (currentIntensity < 0.3) {
@@ -1758,6 +1786,13 @@ function playMusicBar() {
       // Full random for maximum variety
       activeTrack = pickFrom(ALL_TRACKS)
     }
+    // Smooth crossfade: brief gain dip on track switch to prevent abrupt transition
+    if (activeTrack !== prevTrack && musicGain) {
+      const curVol = musicGain.gain.value
+      musicGain.gain.setValueAtTime(curVol, now)
+      musicGain.gain.linearRampToValueAtTime(curVol * 0.4, now + 0.05)
+      musicGain.gain.linearRampToValueAtTime(curVol, now + 0.15)
+    }
   }
 
   // Chord pad for the whole bar