feat(notifications): add notification sound playback for new alerts and include sound file documentation

public/sounds/README.md

@@ -0,0 +1,13 @@
+# Notification Sound
+
+Pop-up Sound by BeezleFM -- https://freesound.org/s/512135/ -- License: Attribution 4.0
+
+Place your notification sound file here as:
+
+  `notification.mp3`
+
+The file is served at `/sounds/notification.mp3` and played automatically
+whenever a new in-app notification arrives.
+
+Supported formats: MP3, OGG, WAV — MP3 recommended for broadest browser support.
+Keep the file short (< 2 s) and not too loud.