Use relative asset and API URLs so the UI works under a path prefix

index.html and app.js requested /static/... and /api/... from the site
root, so serving the app behind a reverse proxy at a sub-path (e.g.
https://host/uj/ with the prefix stripped) 404'd every asset and call.
Relative URLs resolve against the page, which is the same at / and at
/uj/; direct access is unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015UXA39Sh9coAx4FhRkd8R2

static/index.html

@@ -4,8 +4,8 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Audio Mastering Analysis Toolkit</title>
-<link rel="stylesheet" href="/static/vendor/uPlot.min.css">
-<link rel="stylesheet" href="/static/app.css">
+<link rel="stylesheet" href="static/vendor/uPlot.min.css">
+<link rel="stylesheet" href="static/app.css">
 </head>
 <body>
 
@@ -89,9 +89,9 @@
 
 <div id="drop-overlay" class="drop-overlay" hidden><span>Drop to analyse</span></div>
 
-<script src="/static/vendor/uPlot.iife.min.js"></script>
+<script src="static/vendor/uPlot.iife.min.js"></script>
 <!-- app.js is an ES module: it imports ./audio-store.js, which owns the
      browser-side copy of every uploaded file. -->
-<script type="module" src="/static/app.js"></script>
+<script type="module" src="static/app.js"></script>
 </body>
 </html>