fix: IPv6-safe virtual host port stripping via net.SplitHostPort

[Copilot]

Both stripDownHostPort (down.go) and stripPort (virtual_host.go) failed to strip ports from bracketed IPv6 hosts — [::1]:5244 was returned unchanged, causing all IPv6 vhost lookups to miss.

Changes

• server/middlewares/helpers.go (new): Shared stripHostPort helper using net.SplitHostPort, which handles IPv4, bracketed IPv6, and port-free hosts correctly
• down.go: Remove stripDownHostPort; use stripHostPort
• virtual_host.go: Remove stripPort and unused strings import; use stripHostPort

// Before — [::1]:5244 returns unchanged (bracket detected, port not stripped)
func stripPort(host string) string {
    if idx := strings.LastIndex(host, ":"); idx != -1 {
        if !strings.Contains(host, "[") {
            return host[:idx]
        }
    }
    return host
}

// After — delegates to net.SplitHostPort; [::1]:5244 → ::1
func stripHostPort(host string) string {
    h, _, err := net.SplitHostPort(host)
    if err != nil {
        return host
    }
    return h
}

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