feat: implement notification sound functionality with user customization options

docs/articles/notification-sounds.md

@@ -0,0 +1,43 @@
+# Notification Sounds
+
+EchoHub can play a notification sound when someone @mentions you. This is **disabled by default** and must be enabled in your profile settings.
+
+## Enabling Notifications
+
+Open your profile (`/profile`) and check the **"Notification sound on @mention"** checkbox, then save. You can also adjust the **Volume** (0-100, default 30). All settings are persisted in `~/.echohub/config.json`.
+
+## Customizing the Sound
+
+The client ships with a default `Notification.mp3` in the `Assets` folder. To use your own notification sound, replace the file at:
+
+```
+<app-directory>/Assets/Notification.mp3
+```
+
+The file must be a valid `.mp3` or `.wav` audio file. The replacement takes effect on the next app launch.
+
+Alternatively, set a custom path in `~/.echohub/config.json`:
+
+```json
+{
+  "notifications": {
+    "enabled": true,
+    "volume": 30,
+    "soundFile": "/path/to/your/sound.mp3"
+  }
+}
+```
+
+When `soundFile` is set, EchoHub uses that file instead of the bundled default.
+
+## Disabling Notifications
+
+Uncheck the option in your profile, or edit the config directly:
+
+```json
+{
+  "notifications": {
+    "enabled": false
+  }
+}
+```