Set up Storybook for widget components

[PAMulligan]

Goal

Stand up Storybook so designers and contributors can iterate on widget components in isolation, and so we can publish a hosted reference.

Acceptance criteria

• Storybook 8.x configured for the widget package
• Stories for every public component
• Theming addon hooked into Storybook controls
• Static build deployed to a preview URL on every PR
• CI publishes the main-branch Storybook to GitHub Pages

Milestone

v1.2.0