Fall back to the request Host for the join-token panel_addr

[iamfarhad]

IP-only deployments (no PANEL_DOMAIN configured) got panel_addr: null, so the join-token dialog showed no control-plane address exactly where the guidance matters most. The Host the admin is browsing the panel on reaches this server by definition - use it (port stripped) when neither the live panel-domain setting nor the boot-time PANEL_DOMAIN is set.

[gemini-code-assist]

Code Review

This pull request updates the control plane address resolution to fall back to the request's Host header when no panel domain is configured, facilitating IP-only deployments. The API documentation has been updated accordingly. Feedback is provided regarding a potential issue with IPv6 addresses without a port, which could result in double-bracket wrapping when calling net.JoinHostPort; a code suggestion is included to strip the brackets if they are present.

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[gemini-code-assist]

If the admin accesses the panel using an IPv6 address without a port (e.g., [::1]), net.SplitHostPort will fail and host will be set to [::1]. When net.JoinHostPort is subsequently called, it will detect the colon in [::1] and wrap it in brackets again, resulting in an invalid address like [[::1]]:port.

To prevent this double-wrapping, strip the brackets from host if they are present before calling net.JoinHostPort.

Suggested change

	if host == "" {
	if h, _, err := net.SplitHostPort(r.Host); err == nil {
	host = h
	} else {
	host = r.Host
	}
	}
	if host == "" {
	if h, _, err := net.SplitHostPort(r.Host); err == nil {
	host = h
	} else {
	host = r.Host
	}
	if len(host) > 2 && host[0] == '[' && host[len(host)-1] == ']' {
	host = host[1 : len(host)-1]
	}
	}