Let the message interaction configure its page title

The message interaction shows a configured message while a step is parked, but its page title is always the dispatcher default "Continue" (or, since #3606872: Let each interaction provide its own dispatcher page title, whatever the interaction returns in code). A site building a flow from message steps cannot title each one (for example "Booking received" on a confirmation step).

Proposed resolution

Add an optional "Page title" setting to the message interaction. When set, the interaction title() returns it so the dispatcher uses it as the page title; when empty, the default is kept. This widens the interaction title() contract added in #3606872: Let each interaction provide its own dispatcher page title to allow a plain admin-configured string in addition to a fixed translatable label.

Remaining tasks

Review the merge request.