Email templates: edit, preview against an entry, and pick one in a workflow

The console half of email templates as content in barakoCMS (filed the same day, milestone 4.3.0).

## Change

- **Email templates screen.** A list of templates with where each is used (which workflows). An editor with Subject, a markdown Body using the writing editor from #138, and Layout.
- **Placeholder picker.** Inserts `{{data.Field}}`, `{{createdBy.email}}`, `{{transition.at | date "h:mm tt"}}` and the rest from the workflow's available variables, chosen for a content type, so nothing is typed by hand.
- **Preview.** Pick an entry of that type and see the rendered subject and HTML, at phone and desktop width, beside the editor. Test send to yourself.
- **Workflow action.** The Email action's form offers a template picker, with To still a field that takes placeholders. Existing actions with Subject and Body keep working and offer "Save as template".
- **Layouts.** A small screen for header, footer and colours, defaulting to the Site settings (#134).

## Done when

- A non-developer rewords a template, previews it against a real entry and saves, without opening a workflow.

Start with the existing Email action form and the writing editor referenced by #138; also review Site settings referenced by #134. Map the template list/editor, placeholder picker, entry-based preview, layout screen, and workflow integration, then verify that a non-developer can reword, preview, and save a template without opening a workflow.

Written by the indexing model from the issue text.