WebComponent: Implement ClickToSign behavior and rendering

[mikewolfd]

Motivation

The webcomponent package needs to render ClickToSign so forms with click-to-sign attestation work in the browser.

Scope

• Add useClickToSign behavior hook in packages/formspec-webcomponent/src/behaviors/
  • Reads the bound group's children (accepted, signedBy, signedAt, statement)
  • On sign click: sets accepted=true, signedBy to provided/typed name, signedAt to current ISO datetime, statement to the configured text
  • On revoke (if revocable): clears all children back to defaults
  • Exposes reactive state for signed/unsigned rendering
• Add ClickToSign to the default adapter component registry
• Add ClickToSign rendering in the default adapter (display statement text, sign button, signed state display)
• Add fallback logic: Core processors replace ClickToSign with Stack + Text + Toggle + readonly fields
• Add story to packages/formspec-webcomponent/stories/
• Add E2E test covering sign, revoke, validation, and readonly states

Acceptance Criteria

• ClickToSign renders and captures structured attestation data
• Signed state displays who signed and when
• Revoke works when enabled
• Required validation works (accepted must be true)
• Readonly state prevents signing/revoking
• Fallback produces a functional Stack + Toggle layout
• Story demonstrates the component
• E2E test passes

Dependencies

Depends on #3 (schema must validate the component nodes).