fix(mobile): move baseUrl config to experiments.baseUrl and filter static assets in express wildcard handler

backend/src/index.ts

@@ -49,6 +49,14 @@ app.get('/{*path}', (req, res, next) => {
   if (req.path.startsWith(apiRoot)) return next();
   if (req.path === '/_cron/tick') return next();
   
+  // Do not serve HTML for static assets or requests that do not accept HTML
+  const isAsset = /\.[a-z0-9]{2,4}$/i.test(req.path);
+  const acceptsHtml = req.headers.accept?.includes('text/html');
+  if (isAsset || !acceptsHtml) {
+    res.status(404).json({ success: false, error: 'Not found' });
+    return;
+  }
+  
   if (req.path.startsWith('/mobile')) {
     res.sendFile(path.join(publicDir, 'mobile/index.html'), (err) => {
       if (err) next();