Add `textarea` component (React + Angular)

[sjoerdbeentjes]

Part of #43.

React (@surfnet/react)

• Vendored @shadcn/textarea — Textarea (auto-sizes via field-sizing-content)
• textareaContract added to @surfnet/contracts (description-only)
• Storybook stories: Default, Disabled, Invalid

Angular (@surfnet/angular)

• Vendor Spartan textarea via CLI
• Wire to textareaContract
• Storybook story matching React coverage