Studio: Add ClickToSign to authoring palette and scaffolding

[mikewolfd]

Motivation

Form authors need to add click-to-sign attestation fields through the Studio authoring UI without manually constructing the group+children pattern.

Scope

• Add "Click to Sign" to the AddItemPalette in packages/formspec-studio/src/components/AddItemPalette.tsx
• Implement scaffolding in studio-core helpers: when the author adds a ClickToSign, auto-create the group with the four standard children (accepted, signedBy, signedAt, statement)
• Add ClickToSign to sampleFieldValue / generateDefinitionSampleData in packages/formspec-studio-core/src/mapping-sample-data.ts — generate realistic attestation sample data for mapping previews
• Add ClickToSign component node scaffolding when a component document exists

Acceptance Criteria

• AddItemPalette includes a "Click to Sign" option
• Selecting it creates the correct group+children structure
• Sample data generation produces realistic attestation data
• Studio tests pass

Dependencies

Depends on #4 (component must be renderable for preview).