feat: support port=0 for auto-assign free port

[otakenz]

What

Allows port = 0 in config to auto-assign any free port. When port is 0:

• Skips the port-in-use check (meaningless for auto-assign)
• Reads back the actual assigned port via uv_tcp_getsockname()
• Uses the actual port in browser URLs instead of the config value

Changes

• server/init.lua — call getsockname() after bind(), store self.port
• lua/livepreview/init.lua — wrap port-in-use check in if port ~= 0, return actual port from start(), use it in pick()
• plugin/livepreview.lua — use lp.serverObj.port for browser URL in :LivePreview start

Why

Currently if port 8080 (or any fixed port) is in use by another process, the only options are to kill that process or change to a different hardcoded port. With port = 0 the OS picks a free port automatically, no conflict ever.

[github-actions]

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