Feature: Create demo dataset schema validators

[wolfyres]

Closes #169

🎯 Context

This PR introduces pure TypeScript runtime validators to catch malformed demo data before it reaches the preview or publish flows. To ensure zero risk to the core product, all validation logic, custom error types, and tests have been strictly isolated to the demo admin tooling, completely bypassing production mail flows.

🛠️ Key Changes

Created DemoDataValidationError as a custom error type to cleanly aggregate validation failures.

Built formatValidationErrors to transform arrays of validation issues into readable logs for the admin UI.

Implemented validateDemoDataset, a dependency-free runtime schema validator that enforces the expected structure of mock emails and versioning.

Added a comprehensive local test suite (validators.test.ts) using deterministic mock data to verify acceptance and rejection behaviors.

Confirmed all files are scoped exclusively to src/features/demo-admin-dashboard/.