feat: add ability to delete all instruction steps

[maxelkins]

🤖 Created with AI assistance
👨‍💻 Heavily edited, reviewed and steered by @maxelkins

Summary

Teachers editing instructions can now remove every step at once, not just the step they are on. The remove confirmation modal offers a choice of scope, defaulting to the current step so that clearing everything is always a deliberate selection.

Fixes https://github.com/RaspberryPiFoundation/digital-editor-issues/issues/1705

Changes

• Add a scope choice to the remove instructions modal, using the design system Fieldset and RadioInput components
• Default the choice to removing the current step every time the modal opens, and changes the confirm button text depending on the selected outcome
• Hide the choice when there is only one step, where the button reads "Remove instructions"
• Rewrite the modal copy so it reads correctly whichever scope is chosen
• Scope the modal input styles to text inputs so they no longer override design system form controls, including their focus outlines
• Allow a className to be passed through GeneralModal

Screenshots

Screenshot.2026-08-14.at.17.18.05.mp4

[maxelkins]

Bold radio text is incorrect (shows in standalone) another symptom of react portals moving outside shadow dom.

https://github.com/RaspberryPiFoundation/digital-editor-issues/issues/1716