Add working sound effects and background music via Web Audio API

Synthesized audio system with 9 distinct SFX (click, success, warning,
danger, purchase, achievement, era transition, info) mapped to all game
notifications, plus generative ambient background music with chord
progressions. Adds SFX volume slider to settings alongside existing
music volume control. No audio files or npm dependencies needed.

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

apps/web/src/audio/index.ts

@@ -0,0 +1,69 @@
+import { AudioManager } from './AudioManager';
+import type { GameSettings } from '@token-empire/shared';
+import { useGameStore } from '@/store';
+
+export { AudioManager } from './AudioManager';
+export { triggerNotificationSound, playUISound } from './sounds';
+export type { SoundId } from './synthesizer';
+
+function applyVolumes(audio: AudioManager, settings: GameSettings): void {
+  audio.setSoundEnabled(settings.soundEnabled);
+  audio.setMusicVolume(settings.musicVolume);
+  audio.setSfxVolume(settings.sfxVolume ?? 0.5);
+}
+
+function shouldPlayMusic(settings: GameSettings): boolean {
+  return settings.soundEnabled && settings.musicVolume > 0;
+}
+
+export function initAudioSystem(): () => void {
+  const audio = AudioManager.getInstance();
+  let gestureListenerActive = false;
+
+  const settings = useGameStore.getState().meta.settings;
+  applyVolumes(audio, settings);
+
+  // Music requires a user gesture to start (browser autoplay policy).
+  // We register a one-shot listener that starts music on first click/key.
+  const startOnGesture = () => {
+    const s = useGameStore.getState().meta.settings;
+    if (shouldPlayMusic(s)) {
+      audio.startMusic();
+    }
+    document.removeEventListener('click', startOnGesture);
+    document.removeEventListener('keydown', startOnGesture);
+    gestureListenerActive = false;
+  };
+
+  if (shouldPlayMusic(settings)) {
+    document.addEventListener('click', startOnGesture);
+    document.addEventListener('keydown', startOnGesture);
+    gestureListenerActive = true;
+  }
+
+  const unsub = useGameStore.subscribe((state) => {
+    const next = state.meta.settings;
+    applyVolumes(audio, next);
+
+    if (shouldPlayMusic(next)) {
+      if (!audio.isMusicPlaying) {
+        if (audio.isContextActive) {
+          audio.startMusic();
+        } else if (!gestureListenerActive) {
+          document.addEventListener('click', startOnGesture);
+          document.addEventListener('keydown', startOnGesture);
+          gestureListenerActive = true;
+        }
+      }
+    } else {
+      audio.stopMusic();
+    }
+  });
+
+  return () => {
+    unsub();
+    document.removeEventListener('click', startOnGesture);
+    document.removeEventListener('keydown', startOnGesture);
+    audio.dispose();
+  };
+}