EDITOR_SRC: string = ...

The iframe src. Only the /editor path is constant (ADR 0015). We pass the HOST's own origin as ?host= so the guest knows where to post texbrain:ready and which origin to accept messages from. This is essential in prod: the host page is a custom scheme (tauri://localhost) and the referrer to a texbrain:// iframe is stripped/opaque, so the guest's old referrer-derived origin was wrong and the bridge silently never handed off. The host knows its own origin unambiguously via window.location.origin.