fix(frontend): use relative URLs to support reverse proxy subpath (/sync/)

app/templates/index.html

@@ -4,7 +4,7 @@
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>Navidrome Music Sync</title>
-  <link rel="stylesheet" href="/static/style.css">
+  <link rel="stylesheet" href="static/style.css">
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
   <link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;500;600;700&display=swap" rel="stylesheet">
@@ -83,6 +83,6 @@
     </footer>
   </div>
 
-  <script src="/static/app.js"></script>
+  <script src="static/app.js"></script>
 </body>
 </html>