fix(ui-server): auto-fallback to a random port on EADDRINUSE (#82)

[xiaohongrsx]

Summary

Fixes #82 — the auto-install script aborts when ports 3000/3001 are already in use.

The online installer launches the Web UI via npm run start:built, which runs ui/server/index.js directly. That code path called server.listen() with no error handler, so a busy port crashed the process with an uncaught EADDRINUSE exception and aborted the installation instead of recovering.

Changes

ui/server/index.js:

• Add listenWithPortFallback(): on EADDRINUSE, retry on a random high port (20000–59999), up to 5 attempts, then exit with a clear, actionable message.
• Random ports are used instead of preferred + 1 because adjacent ports are frequently held by the same multi-port application that already took the preferred one.
• The actually-bound port is now used for the "Server Ready" log line and the browser auto-open.
• The bound port is written back to process.env.SERVER_PORT so modules that self-reference it (e.g. routes/taskmaster.js) hit the correct port after a fallback.
• Removed the pre-bind "production mode" log line that printed the preferred (possibly wrong) port.

Behavior

• Preferred port free → binds it as before (no behavior change).
• Preferred port busy → logs a warning, retries on a random high port, prints the real URL.
• 5 failed attempts → exits with guidance to set SERVER_PORT manually.

Testing

Verified the fallback logic in isolation:

• Preferred port free → binds preferred port. ✅
• Preferred port occupied → falls back to a random high port and binds successfully. ✅

Non-EADDRINUSE errors (bad host, permissions) still reject and are not silently retried.