feat: signal when chat runs complete

Users can miss chat completions while the app is in the background.

They can also miss completions when their attention is elsewhere.

Add opt-out sound notifications and a temporary title marker.

This makes completion noticeable without external audio assets or persistent browser notifications.

src/utils/notificationSound.ts

@@ -0,0 +1,83 @@
+const NOTIFICATION_SOUND_ENABLED_STORAGE_KEY = 'notificationSoundEnabled';
+const AudioContextConstructor =
+  typeof window !== 'undefined'
+    ? window.AudioContext || (window as typeof window & { webkitAudioContext?: typeof AudioContext }).webkitAudioContext
+    : undefined;
+
+let audioContext: AudioContext | null = null;
+
+export const isNotificationSoundEnabled = (): boolean => {
+  if (typeof localStorage === 'undefined') {
+    return true;
+  }
+
+  return localStorage.getItem(NOTIFICATION_SOUND_ENABLED_STORAGE_KEY) !== 'false';
+};
+
+export const setNotificationSoundEnabled = (enabled: boolean): void => {
+  if (typeof localStorage === 'undefined') {
+    return;
+  }
+
+  localStorage.setItem(NOTIFICATION_SOUND_ENABLED_STORAGE_KEY, String(enabled));
+};
+
+const getAudioContext = (): AudioContext | null => {
+  if (!AudioContextConstructor) {
+    return null;
+  }
+
+  if (!audioContext) {
+    audioContext = new AudioContextConstructor();
+  }
+
+  return audioContext;
+};
+
+const playTone = (
+  context: AudioContext,
+  frequency: number,
+  startsAt: number,
+  duration: number,
+  peakVolume: number,
+): void => {
+  const oscillator = context.createOscillator();
+  const gain = context.createGain();
+
+  oscillator.type = 'sine';
+  oscillator.frequency.setValueAtTime(frequency, startsAt);
+
+  // Shape the volume so the synthesized tone starts and stops cleanly.
+  gain.gain.setValueAtTime(0.0001, startsAt);
+  gain.gain.exponentialRampToValueAtTime(peakVolume, startsAt + 0.015);
+  gain.gain.exponentialRampToValueAtTime(0.0001, startsAt + duration);
+
+  oscillator.connect(gain);
+  gain.connect(context.destination);
+  oscillator.start(startsAt);
+  oscillator.stop(startsAt + duration + 0.02);
+};
+
+export const playChatCompletionSound = async ({ force = false } = {}): Promise<void> => {
+  if (!force && !isNotificationSoundEnabled()) {
+    return;
+  }
+
+  const context = getAudioContext();
+  if (!context) {
+    return;
+  }
+
+  try {
+    if (context.state === 'suspended') {
+      await context.resume();
+    }
+
+    const now = context.currentTime;
+    playTone(context, 740, now, 0.12, 0.075);
+    playTone(context, 988, now + 0.11, 0.16, 0.06);
+  } catch (error) {
+    // Browsers may block audio until the page receives a user gesture.
+    console.warn('Unable to play notification sound:', error);
+  }
+};