Show the underlying cause when Seafile is unreachable

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server.js

@@ -51,7 +51,9 @@ async function proxySeafile(req, res) {
     if (upstream.body) Readable.fromWeb(upstream.body).pipe(res); else res.end();
   } catch (e) {
     res.writeHead(502, { 'content-type': 'application/json' });
-    res.end(JSON.stringify({ error: 'Seafile unreachable: ' + e.message }));
+    const why = e.cause ? (e.cause.code || e.cause.message) : '';
+    console.error('Seafile proxy:', target.split('?')[0], e.message, why);
+    res.end(JSON.stringify({ error: 'Seafile unreachable: ' + e.message + (why ? ' (' + why + ')' : '') }));
   }
 }