[client] Compare service URLs as endpoints, not as strings

Three places in one request path each had their own notion of "same
management URL": the config layer compared the parsed URLs as strings, the
privileged-change gate compared scheme + host + effective port, and the MDM
conflict check compared strings after filling in the default port. Only the
middle one was right.

A string comparison answers the wrong question. "https://api.netbird.io",
"https://api.netbird.io/" and "https://API.netbird.io:443" are one endpoint
written three ways, so a client restating its own management URL with a
trailing slash — a normal way to write it — was still read as a client asking
to be repointed, and the update-settings gate refused it. The MDM check had
the same flaw against the enforced value.

profilemanager.SameServiceURL is now the single comparison: same scheme, same
host case-insensitively as DNS names are, same effective port. The config
layer, the privileged-change gate and the MDM conflict check all defer to it,
so there is one answer to "did this URL change?" instead of three.

client/server/ssh_gate.go

@@ -331,21 +331,5 @@ func sameManagementURL(stored *url.URL, requested string) bool {
 		return false
 	}
 
-	return stored.Scheme == parsed.Scheme &&
-		stored.Hostname() == parsed.Hostname() &&
-		effectivePort(stored) == effectivePort(parsed)
-}
-
-func effectivePort(u *url.URL) string {
-	if port := u.Port(); port != "" {
-		return port
-	}
-	switch u.Scheme {
-	case "https":
-		return "443"
-	case "http":
-		return "80"
-	default:
-		return ""
-	}
+	return profilemanager.SameServiceURL(stored, parsed)
 }