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/i18n/locales/en/settings.json

@@ -110,6 +110,12 @@
       "unsupported": "Push notifications are not supported in this browser.",
       "denied": "Push notifications are blocked. Please allow them in your browser settings."
     },
+    "sound": {
+      "title": "Sound",
+      "description": "Play a short tone when a chat run finishes.",
+      "enabled": "Enabled",
+      "test": "Test sound"
+    },
     "events": {
       "title": "Event Types",
       "actionRequired": "Action required",