Method OnJsReady
OnJsReady()
Called from JS (App.tsx mount effect) once all __grindfest callbacks are registered. Returns the name of the pending panel to open ("selection" | "creation" | "settings" | null) so JS can dispatch directly — we can't re-enter ScriptEngine.Eval() from inside a JS callback. Also resets _jsReady (it gets set again at the end of this call) so a JS context Reload (which keeps the static field but discards globals) is handled correctly.
public static string OnJsReady()