Add draft dataset JSON import mapper

[eleven-smg]

Closes #272

Adds a campaign/draft dataset JSON import mapper for the demo admin dashboard. It is the inverse of the existing dataset JSON export (#190): it takes an imported JSON payload and maps it into review-safe drafts that can be loaded into the draft dataset store.

What's included (all under src/features/demo-admin-dashboard/):

• types/datasetImport.ts — DatasetImportResult and DatasetImportIssue types
• helpers/datasetImport.ts — mapImportedDataset() and parseDatasetImport()
• tests/datasetImport.test.ts — 8 unit tests
• docs/DATASET_JSON_IMPORT.md — usage and validation rules

Behaviour:

• Validates the envelope shape and schema version (DATASET_EXPORT_SCHEMA_VERSION)
• Validates each draft (non-empty id, string subject/body, recipients array)
• Enforces safe demo recipient addresses (example.com / example.org / *stealth.demo)
• Rejects duplicate draft ids and mismatched count
• Strips unknown fields from imported drafts
• Atomic import: if any issue is found, no drafts are returned, with a list of human-readable errors
• All data stays fake, deterministic, and safe for public review; no files outside the feature folder were changed

Checks run locally: prettier, tsc --noEmit, eslint (0 errors), vitest (8 passed), and build all pass.