Add dataset validation results panel to demo admin dashboard

[eleven-smg]

Closes #220

Summary

Adds a ValidationResultsPanel to the demo admin dashboard that surfaces dataset validation errors and warnings, each with its field path, so demo data issues are easy to scan and fix.

All work is contained within src/features/demo-admin-dashboard/. No files outside that folder are touched.

What's included

• Component: ValidationResultsPanel.tsx — groups issues by severity (errors → warnings → info), shows per-severity counts, field paths, and optional fix hints, with a clean empty state when there are no issues.
• Types: validation-types.ts — ValidationIssue, ValidationSeverity, and ValidationNavigation (datasetId / recordId / fieldPath) for jumping to a source field.
• Helpers: validation.ts — summarizeValidation, sortIssues, groupBySeverity, getIssueNavigation, isDatasetValid.
• Demo data: validationFixtures.ts — deterministic, fake sample issues (no real data, secrets, or network calls).
• Docs: ValidationResultsPanel.md.
• Tests: validation.test.ts — covers counts, sorting, grouping, navigation metadata, and validity checks.

Navigation metadata

Passing onSelectIssue makes each issue clickable and reports { datasetId, recordId, fieldPath } so a parent view can navigate to the offending field.

Testing

• npx vitest run src/features/demo-admin-dashboard/validation.test.ts — all passing
• Type-check clean for the new files