fix: convert ES modules to classic scripts for file:// compatibility

Browsers block ES module imports over the file:// protocol due to CORS.
Users opening the overlay by double-clicking the HTML file saw all JS
fail to load. Replace import/export with a window.OBS global namespace
and classic <script> tags so the overlay works without a local server.

Made-with: Cursor

js/audio-controller.js

@@ -10,7 +10,7 @@
  * @property {HTMLInputElement} soundUrl
  */
 
-export class AudioController {
+class AudioController {
   /** @type {HTMLAudioElement | null} */
   #audio = null;
 
@@ -137,3 +137,6 @@ export class AudioController {
     }, intervalMs);
   }
 }
+
+window.OBS = window.OBS || {};
+window.OBS.AudioController = AudioController;