Ask for a different backend port when the default one is taken

[feanor91]

Summary

• The desktop app used to crash (or silently hang ~60s) if something else on the machine already held the backend's port. It now checks before spawning the backend, and lets the user pick a free port instead — defaulting to one it finds automatically.
• Also fixes the free-port check itself: it was probing 127.0.0.1 specifically, which can report "free" even when the backend's actual no-host dual-stack bind would fail (observed with Docker Desktop's port-forwarding on Windows).

Test plan

• Built the frontend (npm run build) and launched the packaged Electron app (electron .) locally with port 3001 already occupied by another process.
• Confirmed the port-conflict prompt appeared, offered a free port (3002), and the backend started successfully on the confirmed port after accepting it.
• Confirmed the app shuts down cleanly (backend receives SIGTERM) on window close.

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