feat(ui): implement dynamic port allocation for lean-spec ui (spec 172)

[nettee]

• Add automatic port detection when default port is in use
• Implement findAvailablePort() utility function
• Update UI launcher to resolve port before server startup
• Add clear user feedback when port is automatically changed
• Update documentation with port allocation behavior

[tikazyq]

@nettee thanks for your PR! But unfortunately I've cleaned up the rust target files so I had to force push some commits to the repo. Would you clone the latest GitHub lean-spec repo and start from there? Otherwise, it'll be very hard to merge into the current main branch.

[nettee]

@tikazyq I've created a new PR based on the current main branch:
#102