Electron app: 502 error when launching terminal app for a running session

## Problem

When using the Electron desktop app to log into a Backend.AI cluster and launch the **terminal** app for a running session, a new window opens and displays a **502 error**.

## Reproduction

1. Open the Electron app
1. Log in to the cluster
1. Open a running session
1. Launch the terminal app

A new window opens with a 502 error.

## Expected

The terminal app should open and connect successfully (as it does in the browser).

## Actual

New window opens showing a 502 error.

## Notes

- Reproduces only in the **Electron app**.
- Accessing the same cluster via a **web browser** works correctly — the terminal app launches without error.
- Suggests an issue with the Electron-bundled wsproxy / app routing for terminal sessions, not the cluster itself.

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Captured while working on branch: main

JIRA Issue: FR-2798