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/de/settings.json

@@ -97,6 +97,14 @@
     "plugins": "Plugins",
     "about": "Info"
   },
+  "notifications": {
+    "sound": {
+      "title": "Ton",
+      "description": "Spielt einen kurzen Ton ab, wenn ein Chat-Lauf abgeschlossen ist.",
+      "enabled": "Aktiviert",
+      "test": "Ton testen"
+    }
+  },
   "appearanceSettings": {
     "darkMode": {
       "label": "Darkmode",