refactor: extract shared dataset types to src/types/datasets.ts

[devin-ai-integration]

Summary

The DatasetField, DroppedField, and DatasetDetail type definitions are currently duplicated in:

• src/pages/index.tsx (as type aliases)
• src/__tests__/datasets.test.ts (as interface declarations)

If the JSON schema evolves (e.g., a new required field), both locations need to be updated independently.

Proposed Change

Extract these types into a shared module at src/types/datasets.ts and import from both the page component and the test file.

Context

Identified during PR #8 review (Devin Review comment).