MIGHTY FORM-DRIVEN ACCORDION TEMPLATE

What changed in this pass:
- Keeps the interactive fully form-driven via mighty-form-config.json.
- Ensures the block is marked complete when all accordion sections have been visited.
- Keeps rich text authoring support through Mighty richtext-input fields.
- Adds three generic starter sections by default.
- Uses red, amber, and green only as starter colours; authors can change each section to any colour.
- Supports custom header text colour and optional images per section.
- Restores the white card plus inset coloured bar styling pattern.

How authors use it:
1. Open the interactive block in Mighty Studio.
2. Edit the page heading, lede, instruction, and accordion sections in the form.
3. Add, remove, reorder, and customize section colours, header text colours, images, and rich text.
4. Publish as normal.
