fix: exit with the real error when the backend cannot bind its port (supersedes #14)

[abubear]

Supersedes #14 by @srbot2016, which no longer merges: main has since gained a
server.on('error') handler of its own (209154b), so the PR conflicts and its
const server = app.listen(...) change is already there. The part of #14 that
still matters is carried here, with credit to the original author on the commit.

The bug that is still on main

server.on('error', (err) => {
  if (err.code !== 'EADDRINUSE') throw err;
  ...
});

A throw inside an 'error' listener becomes an uncaught exception, and
process.on('uncaughtException') at the top of server.js deliberately keeps
this process alive. So any bind failure other than a port collision — @srbot2016
hit listen EACCES: permission denied 0.0.0.0:3001 — leaves a backend process
running that never started listening. The Electron shell waits, times out, and
reports:

Backend startup failed: Backend did not start in time

The real Node.js error is printed only as "Uncaught exception (server kept
alive)" noise, and the process lingers instead of failing.

Reproduced on main (Windows, port inside a WinNAT-reserved range): the process
was still alive 60s later, having never bound.

The change

Every listen failure is fatal and named, then process.exit(1):

• EADDRINUSE — unchanged message and advice.
• EACCES — new: says permission denied, points at privileged ports (<1024) and
  at Windows reserved ranges (netsh int ipv4 show excludedportrange protocol=tcp).
• anything else — prints the error object itself, so the cause is in the log
  rather than behind a timeout.

Nothing is swallowed: the unknown branch logs the full error, and the exit code
now tells the shell, a container, or CI that the start failed.

Testing

npm test does not exist in this repo (no test script or runner), so this was
verified by running the backend:

case	how	result
normal start	PORT=3201 npm start	banner prints, server stays up
EADDRINUSE	squat the port, then start on it	message + advice, exit code 1
EACCES	PORT=49726 (WinNAT-reserved range)	permission-denied message + advice, exit code 1
EACCES on main (before)	same	logged as an uncaught exception, process alive after 60s, never listening

npx eslint server.js — clean. Repo-wide npm run lint still fails on
pre-existing react-hooks issues under src/, untouched by this PR.

Note for the maintainer

#14 can be closed once this merges — the fix and the credit both live here.