isValidNetworkLayerUrl

Whether url is one MapLayersManager.addNetworkMapLayer will accept: an explicit, parseable http(s) URL.

The scheme check is on the string, not the parsed protocol — Ktor's Url defaults a missing scheme to http, so example.com/map.kml would parse as valid and then be stored as a string nothing can fetch. Shared with the add-layer dialog so the form and the store cannot disagree about what is acceptable.