Fix Android media notification: add setPositionState + seek handlers

- setPositionState synced every second via updateProgress so the notification shows progress bar and timestamps
- seekto handler enables dragging the notification progress bar
- seekbackward/seekforward handlers enable 10s skip buttons

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v15';
+const CACHE = 'ns-v16';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'sw.js'];
 const SKIP_URLS = ['/rest/stream', '/rest/download', '/config.js'];