fix: remote proxy strips /api prefix causing remote Sencho to return SPA HTML

app.use('/api/', ...) causes Express to strip the '/api/' prefix from
req.url before http-proxy-middleware sees it, so the proxy was forwarding
'/stats' to the remote instead of '/api/stats'. The remote Sencho found
no matching route, fell through to its SPA catch-all, and returned
index.html (200 text/html) — explaining the SyntaxError on res.json().

Added pathRewrite: (path) => '/api' + path to restore the full path.
The connection test passed because testRemoteConnection calls the remote
directly via axios with the full URL, bypassing the proxy entirely.

backend/src/index.ts

@@ -331,6 +331,10 @@ const remoteNodeProxy = createProxyMiddleware<Request, Response>({
     const node = NodeRegistry.getInstance().getNode(req.nodeId);
     return node?.api_url?.replace(/\/$/, '');
   },
+  // When mounted at app.use('/api/', ...), Express strips the '/api/' prefix from
+  // req.url before the middleware sees it. Re-add it so the remote Sencho instance
+  // receives the full path (e.g. '/stats' becomes '/api/stats').
+  pathRewrite: (path) => '/api' + path,
   on: {
     proxyReq: (proxyReq, req) => {
       const node = NodeRegistry.getInstance().getNode(req.nodeId);