fix: guard Notification API for unsupported browsers

Check typeof Notification before accessing .permission or
requestPermission(). Fixes crash on browsers/contexts where
the Notification API is unavailable (e.g. some WebViews).

src/lib/i18n.ts

@@ -28,6 +28,8 @@ const en = {
   'header.logout': 'Logout',
   'header.toggleSidebar': 'Toggle sidebar',
   'header.changeLanguage': 'Change language',
+  'header.soundOn': 'Enable notification sound',
+  'header.soundOff': 'Disable notification sound',
 
   // Chat
   'chat.welcome': 'PinchChat',
@@ -106,6 +108,8 @@ const fr: Record<keyof typeof en, string> = {
   'header.logout': 'Déconnexion',
   'header.toggleSidebar': 'Afficher/masquer la barre latérale',
   'header.changeLanguage': 'Changer de langue',
+  'header.soundOn': 'Activer le son de notification',
+  'header.soundOff': 'Désactiver le son de notification',
 
   'chat.welcome': 'PinchChat',
   'chat.welcomeSub': 'Envoyez un message pour commencer',

src/lib/notificationSound.ts

@@ -0,0 +1,55 @@
+/**
+ * Plays a subtle notification sound using the Web Audio API.
+ * No external audio files needed — synthesized at runtime.
+ *
+ * The sound is a soft two-tone chime (C5 → E5) that's pleasant
+ * and unobtrusive, similar to modern chat apps.
+ */
+
+let audioCtx: AudioContext | null = null;
+
+function getAudioContext(): AudioContext | null {
+  try {
+    if (!audioCtx || audioCtx.state === 'closed') {
+      audioCtx = new AudioContext();
+    }
+    // Resume if suspended (browser autoplay policy)
+    if (audioCtx.state === 'suspended') {
+      audioCtx.resume();
+    }
+    return audioCtx;
+  } catch {
+    return null;
+  }
+}
+
+export function playNotificationSound(volume = 0.3): void {
+  const ctx = getAudioContext();
+  if (!ctx) return;
+
+  const now = ctx.currentTime;
+
+  // Two-tone chime: C5 (523Hz) then E5 (659Hz)
+  const frequencies = [523.25, 659.25];
+  const duration = 0.12;
+  const gap = 0.08;
+
+  frequencies.forEach((freq, i) => {
+    const osc = ctx.createOscillator();
+    const gain = ctx.createGain();
+
+    osc.type = 'sine';
+    osc.frequency.value = freq;
+
+    const start = now + i * (duration + gap);
+    gain.gain.setValueAtTime(0, start);
+    gain.gain.linearRampToValueAtTime(volume, start + 0.01);
+    gain.gain.exponentialRampToValueAtTime(0.001, start + duration);
+
+    osc.connect(gain);
+    gain.connect(ctx.destination);
+
+    osc.start(start);
+    osc.stop(start + duration + 0.01);
+  });
+}